Editor enhancements docs (#2222)

* docs(editor): plugin and schema docs

* docs(editor): add schema docs

* docs(editor): updates

* docs(editor): create new schema docs and sampple

* docs(editor): update articles

* Update _contentTemplates/editor/general.md

Co-authored-by: Dimo Dimov <961014+dimodi@users.noreply.github.com>

* Update _contentTemplates/editor/general.md

Co-authored-by: Dimo Dimov <961014+dimodi@users.noreply.github.com>

* Update _contentTemplates/editor/general.md

Co-authored-by: Iva Stefanova Koevska-Atanasova <koevska@progress.com>

* Update _contentTemplates/editor/general.md

Co-authored-by: Iva Stefanova Koevska-Atanasova <koevska@progress.com>

* Update _contentTemplates/editor/general.md

Co-authored-by: Iva Stefanova Koevska-Atanasova <koevska@progress.com>

* Update components/editor/prosemirror-plugins.md

Co-authored-by: Iva Stefanova Koevska-Atanasova <koevska@progress.com>

* Update components/editor/prosemirror-plugins.md

Co-authored-by: Iva Stefanova Koevska-Atanasova <koevska@progress.com>

* Update components/editor/prosemirror-plugins.md

Co-authored-by: Iva Stefanova Koevska-Atanasova <koevska@progress.com>

* Update components/editor/prosemirror-plugins.md

Co-authored-by: Iva Stefanova Koevska-Atanasova <koevska@progress.com>

* Update _contentTemplates/editor/general.md

Co-authored-by: Dimo Dimov <961014+dimodi@users.noreply.github.com>

* Update _contentTemplates/editor/general.md

Co-authored-by: Dimo Dimov <961014+dimodi@users.noreply.github.com>

* Update components/editor/prosemirror-plugins.md

Co-authored-by: Dimo Dimov <961014+dimodi@users.noreply.github.com>

* Update components/editor/prosemirror-plugins.md

Co-authored-by: Dimo Dimov <961014+dimodi@users.noreply.github.com>

* Update components/editor/prosemirror-schema/create-new-schema.md

Co-authored-by: Dimo Dimov <961014+dimodi@users.noreply.github.com>

* Update components/editor/prosemirror-schema/create-new-schema.md

Co-authored-by: Dimo Dimov <961014+dimodi@users.noreply.github.com>

* Update components/editor/prosemirror-schema/create-new-schema.md

Co-authored-by: Dimo Dimov <961014+dimodi@users.noreply.github.com>

* Update components/editor/prosemirror-schema/create-new-schema.md

Co-authored-by: Dimo Dimov <961014+dimodi@users.noreply.github.com>

* Update components/editor/prosemirror-schema/overview.md

Co-authored-by: Iva Stefanova Koevska-Atanasova <koevska@progress.com>

* Update components/editor/prosemirror-plugins.md

Co-authored-by: Iva Stefanova Koevska-Atanasova <koevska@progress.com>

* Update _contentTemplates/editor/general.md

Co-authored-by: Iva Stefanova Koevska-Atanasova <koevska@progress.com>

* Update components/editor/prosemirror-schema/overview.md

Co-authored-by: Iva Stefanova Koevska-Atanasova <koevska@progress.com>

* Update components/editor/prosemirror-schema/overview.md

Co-authored-by: Iva Stefanova Koevska-Atanasova <koevska@progress.com>

* Update components/editor/prosemirror-schema/overview.md

Co-authored-by: Iva Stefanova Koevska-Atanasova <koevska@progress.com>

* Update components/editor/prosemirror-schema/modify-default-schema.md

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* Update components/editor/prosemirror-schema/modify-default-schema.md

Co-authored-by: Iva Stefanova Koevska-Atanasova <koevska@progress.com>

* Update components/editor/prosemirror-schema/modify-default-schema.md

Co-authored-by: Iva Stefanova Koevska-Atanasova <koevska@progress.com>

* Update components/editor/prosemirror-schema/modify-default-schema.md

Co-authored-by: Iva Stefanova Koevska-Atanasova <koevska@progress.com>

* Update components/editor/prosemirror-schema/create-new-schema.md

Co-authored-by: Iva Stefanova Koevska-Atanasova <koevska@progress.com>

* docs(editor): address feedback

* chore(editor): update samples and links

* chore(editor): address feedback

* chore(editor): update structure and remove repeating info

* chore(editor): minor fixes

* chore(editor): update exteral links

---------

Co-authored-by: Dimo Dimov <961014+dimodi@users.noreply.github.com>
Co-authored-by: Iva Stefanova Koevska-Atanasova <koevska@progress.com>

_config.yml

@@ -216,6 +216,9 @@ navigation:
      "*components/editor/edit-modes":
         title: "Edit Modes"
         position: 80
+     "*components/editor/prosemirror-schema":
+        title: "ProseMirror Schema"
+        position: 120
      "*components/panelbar/data-binding":
         title: "Data Binding"
         position: 2

_contentTemplates/editor/general.md

@@ -11,3 +11,33 @@ This section applies only to Blazor **Server** apps. Blazor **WebAssembly** apps
 Blazor **Server** apps use the **SignalR WebSocket** to send the Editor `Value` from the browser to the server .NET runtime and vice-versa. The default SignalR maximum message size is **32 KB**. To work with larger content (especially paste images as Base64 strings), [increase the max WebSocket message size for the Blazor application]({%slug common-kb-increase-signalr-max-message-size%}).
 
 #end
+#prosemirror-schema-prerequisites
+## Prerequisites
+
+To work with ProseMirror, make sure you are familiar with:
+
+* JavaScript—ProseMirror is a JavaScript library and the schema uses JavaScript syntax.
+* <a href="https://prosemirror.net/docs/guide/#schema" target="_blank">ProseMirror Schema</a>&mdash;The schema structure and its children (<a href="https://prosemirror.net/docs/ref/#model.NodeType" target="_blank">nodes</a> and <a href="https://prosemirror.net/docs/ref/#model.MarkType" target="_blank">marks</a>).
+
+Modifying the ProseMirror Schema is outside of the Editor scope and we do not provide support for such customizations.
+#end
+
+#prosemirror-schema-general-info
+The Editor accepts a custom ProseMirror schema through its `Schema` parameter. Set this `string` parameter to the name of a JavaScript function that:
+
+* Is declared in the global scope (the `window` object).
+* Returns an instance of the <a href="https://prosemirror.net/docs/ref/#model.Schema" target="_blank">ProseMirror `Schema` class</a>(the updated schema). You can access this class from the `ProseMirror` object of the event arguments.
+* Accepts a single argument.
+
+The Editor will call this function and will pass an argument object that contains the following properties:
+
+@[template](/_contentTemplates/common/parameters-table-styles.md#table-layout)
+
+| Property | Description |
+|----------|-------------|
+| `getSchema` | A function that returns the current <a href="https://prosemirror.net/docs/ref/#model.Schema" target="_blank">`Schema` object</a>. Before the Editor is initialized, the returned `Schema` object is the default schema of the Editor. After the Editor is initialized, the returned `Schema` object is the updated schema. If you don't provide a custom schema, this function always returns the default schema. |
+| `getView` | A function that returns the currently used instance of the <a href="https://prosemirror.net/docs/ref/#view.EditorView" target="_blank">`EditorView` object</a>. Before the Editor is initialized, the view (the result of the function) is null. |
+| `ProseMirror` | An object that contains various ProseMirror classes and functions. |
+
+> You can set a custom schema only once during initialization of the Editor component. Further changes to the schema will not take effect and the component will continue using the initial custom or built-in schema.
+#end

components/editor/overview.md

@@ -82,6 +82,10 @@ Images in the content area of the Editor are resizable. To grab the resize handl
 
 The Telerik UI for [Blazor Editor](https://www.telerik.com/blazor-ui/editor) uses the ProseMirror engine and it depends on it. You do not need to add any extra assets or references yourself, though, we have taken care of everything internally.
 
+## ProseMirror Schema and Plugins
+
+The Editor uses a built-in <a href="https://prosemirror.net/docs/guide/#schema" target="_blank">ProseMirror Schema</a> containing some of the most common HTML tags and a set of predefined <a href="https://prosemirror.net/docs/ref/#state.Plugin_System" target="_blank">ProseMirror Plugins</a> for its basic functionalities. You can customize the default ProseMirror [Schema]({%slug editor-prosemirror-schema-overview%}) and [Plugins]({%slug editor-prosemirror-plugins%}) to achieve the desired functionality in the Editor for Blazor. 
+
 ## Editor Parameters
 
 The following table lists Editor parameters, which are not discussed elsewhere in the component documentation. 

components/editor/prosemirror-plugins.md

@@ -0,0 +1,143 @@
+---
+title: ProseMirror Plugins
+page_title:  ProseMirror Plugins
+description: Explore how to use the ProseMirror Plugins in the Editor for Blazor uses.
+slug: editor-prosemirror-plugins
+tags: telerik,blazor,editor,prosemirror,plugins
+published: True
+position: 130
+---
+
+# ProseMirror Plugins
+
+The Telerik UI for Blazor Editor component is based on the <a href="https://prosemirror.net/" target="_blank">ProseMirror library</a>. ProseMirror provides a set of tools and concepts for building rich text editors, using user interface inspired by what-you-see-is-what-you-get.
+
+## Plugins Concept
+
+The ProseMirror <a href="https://prosemirror.net/docs/ref/#state.Plugin_System" target="_blank">plugin system</a> enables developers to create custom tools and functionality. One of the main building blocks of each editor is its <a href="https://prosemirror.net/docs/ref/#state" target="_blank">`EditorState`</a> object. The state is created through a static <a href="https://prosemirror.net/docs/ref/#state.EditorState%5Ecreate" target="_blank">`create`</a> method which takes a configuration object, containing the starting document node, the <a href="https://prosemirror.net/docs/ref/#model.Schema" target="_blank">`Schema`</a>, and a collection of <a href="https://prosemirror.net/docs/ref/#state.Plugin" target="_blank">plugins</a> which will be active in this state.
+
+Plugins are instances of the <a href="https://prosemirror.net/docs/ref/#state.Plugin" target="_blank">`Plugin` class</a> and can model a wide variety of features. The basic ones may only add some <a href="https://prosemirror.net/docs/ref/#view.EditorProps" target="_blank">properties</a> to the Editor view to respond to certain events. More complicated features may add a new state to the editor and update it based on <a href="https://prosemirror.net/docs/ref/#state.Transaction" target="_blank">transactions</a>.
+
+For further details about the ProseMirror plugins, refer to <a href="https://prosemirror.net/docs/guide/#state.plugins" target="_blank">this ProseMirror guide</a>.
+
+Modifying the ProseMirror plugins is outside of the Editor scope and we do not provide support for such customizations.
+
+## Adding a Custom Plugin
+
+ProseMirror is a JavaScript library and the plugins use JavaScript syntax.
+
+To add a custom plugin to the Editor, use the `Plugins` parameter. Set this `string` parameter to the name of a JavaScript function that:
+
+* Is declared in the global scope (`window` object).
+* Returns custom ProseMirror plugins.
+* Accepts a single argument.
+
+The Editor will call this function and will pass an argument object that contains the following properties:
+
+@[template](/_contentTemplates/common/parameters-table-styles.md#table-layout)
+
+| Property | Description |
+|----------|-------------|
+| `getSchema` | A function that returns the current <a href="https://prosemirror.net/docs/ref/#model.Schema" target="_blank">`Schema` object</a>. Before the Editor is initialized, the `Schema` is the default `Schema`. After the Editor is initialized, the returned `Schema` is the updated schema. If you don't provide a custom schema, this function always returns the default schema. |
+| `getView` | A function that returns the currently used instance of the <a href="https://prosemirror.net/docs/ref/#view.EditorView" target="_blank">`EditorView` object</a>. Before the Editor is initialized, the view (the result of the function) is null. |
+| `ProseMirror` | An object that contains various ProseMirror classes and functions.|
+| `getPlugins` | A function that accepts `Schema` as an argument and returns the default Editor plugins. The function must return an array of ProseMirror plugins. |
+
+> To ensure all the built-in functionalities of the Editor are working correctly, the result array must contain the default plugins which can be retrieved by calling the `getPlugins` function.
+
+>caption Adding a Placeholder Plugin
+
+```CSHTML
+@using Telerik.Blazor.Components.Editor
+@inject IJSRuntime js
+
+<TelerikEditor @bind-Value="@EditorValue"
+               Plugins="pluginsProvider"
+               Height="400px"
+               Width="700px">
+</TelerikEditor>
+
+@code {
+    private string EditorValue { get; set; }
+
+    protected override async Task OnAfterRenderAsync(bool firstRender)
+    {
+        if (firstRender)
+        {
+            //Plug the styles with JavaScript as shown below or set the EditMode to `Div` - https://docs.telerik.com/blazor-ui/components/editor/edit-modes/div
+            await js.InvokeVoidAsync("injectEditorStyleTag");
+        }
+    }
+}
+
+@* Move JavaScript code to a separate JS file in production *@
+<script suppress-error="BL9992">
+    window.pluginsProvider = (args) => {
+        const schema = args.getSchema();
+        var placeHolderKey = new args.ProseMirror.PluginKey("placeholder");
+
+        function placeholder(emptyMessage) {
+            console.log(args);
+            return new args.ProseMirror.Plugin({
+                key: placeHolderKey,
+                props: {
+                    decorations: (state) => {
+                        const { doc } = state;
+                        const empty =
+                            doc.textContent === "" &&
+                            doc.childCount <= 1 &&
+                            doc.content.size <= 2;
+                        if (!empty) {
+                            return args.ProseMirror.DecorationSet.empty;
+                        }
+                        const decorations = [];
+                        const decAttrs = {
+                            class: "placeholder",
+                            "data-placeholder": emptyMessage,
+                        };
+                        doc.descendants((node, pos) => {
+                            decorations.push(args.ProseMirror.Decoration.node(pos, pos + node.nodeSize, decAttrs));
+                        });
+                        return args.ProseMirror.DecorationSet.create(doc, decorations);
+                    }
+                }
+            });
+        }
+
+        return [...args.getPlugins(schema), placeholder("Enter some content ...")];
+    }
+
+    function injectEditorStyleTag() {
+        var doc = document.querySelector("iframe").contentWindow.document;
+        var head = doc.querySelector("head");
+
+        var style = doc.createElement("style");
+        style.type = "text/css";
+
+        var iframeStyles = `p.placeholder:first-child:before {
+                            content: attr(data-placeholder);
+                            float: left;
+                            color: rgb(0, 0, 0, 0.5);
+                            cursor: text;
+                            height: 0;
+                            font-style: italic;
+                        }`;
+
+        style.appendChild(doc.createTextNode(iframeStyles));
+        head.appendChild(style);
+    };
+</script>
+```
+
+## See Also
+
+* [Live Demo: Editor - ProseMirror Plugins](https://demos.telerik.com/blazor-ui/editor/prosemirror-plugins)
+* [ProseMirror Schema]({%slug editor-prosemirror-schema-overview%})
+
+
+<!-- # Examples
+
+List here the KB articles created as part of https://github.com/telerik/blazor/issues/9608 
+
+Similar to how this is handle in the Grid State article - https://docs.telerik.com/blazor-ui/components/grid/state#examples
+-->