Merge pull request #6551 from ckeditor/i/6087

Docs: Mentioned the "Deep dive into focus tracking" guide in the "UI library" guide (see #6087).

docs/framework/guides/architecture/ui-library.md

@@ -451,10 +451,12 @@ dropdownView.buttonView.on( 'execute', () => {
 
 ## Keystrokes and focus management
 
-_Coming soon..._
-
 The framework offers built–in classes that help manage keystrokes and focus in the UI. They are particularly useful when it comes to bringing accessibility features to the application.
 
+<info-box>
+	If you want to know how the editor handles focus under the hood and what tools make it possible, check out the {@link framework/guides/deep-dive/focus-tracking **"Deep dive into focus tracking"**} guide.
+</info-box>
+
 ### Focus tracker
 
 The {@link module:utils/focustracker~FocusTracker `FocusTracker`} class can observe a number of HTML elements and determine if one of them is focused either by the user (clicking, typing) or using the `HTMLElement.focus()` DOM method.
@@ -488,6 +490,8 @@ focusTracker.on( 'change:isFocused', ( evt, name, isFocused ) => {
 
 This information is useful when implementing a certain type of UI whose behavior depends on the focus, for example, contextual panels and floating balloons containing forms should hide when the user decides to abandon them.
 
+Learn more about the focus tracker class in the {@link framework/guides/deep-dive/focus-tracking#using-the-focustracker-class "Deep dive into focus tracking"} guide.
+
 ### Keystroke handler
 
 The {@link module:utils/keystrokehandler~KeystrokeHandler `KeystrokeHandler`} listens to the keystroke events fired by an HTML element or any of its descendants and executes pre–defined actions when the keystroke is pressed. Usually, each [view](#views) creates its own keystroke handler instance which takes care of the keystrokes fired by the elements the view has rendered.

docs/framework/guides/custom-editor-creator.md

@@ -121,7 +121,7 @@ mix( MultirootEditor, DataApiMixin );
 
 ## EditorUI class
 
-The `*EditorUI` class is the main UI class which initializes UI components (the main view and the toolbar) and sets up mechanisms like focus tracker or placeholder management. The custom `*EditorUI` class should extend the {@link module:core/editor/editorui~EditorUI base `EditorUI` class} like below:
+The `*EditorUI` class is the main UI class which initializes UI components (the main view and the toolbar) and sets up mechanisms like {@link framework/guides/deep-dive/focus-tracking#using-the-focustracker-class focus tracker} or placeholder management. The custom `*EditorUI` class should extend the {@link module:core/editor/editorui~EditorUI base `EditorUI` class} like below:
 
 ```js
 import EditorUI from '@ckeditor/ckeditor5-core/src/editor/editorui';