monaco devdocs (#15691)

* Create update-monaco-editor.md

* Update and rename update-monaco-editor.md to readme.md

* Update doc/devdocs/modules/powerpreview/monaco/readme.md

Co-authored-by: Heiko <61519853+htcfreek@users.noreply.github.com>

* Update doc/devdocs/modules/powerpreview/monaco/readme.md

Co-authored-by: Heiko <61519853+htcfreek@users.noreply.github.com>

* Update doc/devdocs/modules/powerpreview/monaco/readme.md

Co-authored-by: Heiko <61519853+htcfreek@users.noreply.github.com>

* Update readme.md

* Update readme.md

Co-authored-by: Heiko <61519853+htcfreek@users.noreply.github.com>

doc/devdocs/modules/powerpreview/monaco/readme.md

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+# Developer Preview (Monaco)
+
+Developer preview is based on [Microsofts Monaco Editor](https://microsoft.github.io/monaco-editor/) which is maintained by the Visual Studio Code team.
+
+## Update monaco editor
+
+1. Download Monaco editor with npm: `npm i monaco-editor`.
+2. Delete everything except the `min` folder (the minimised code).
+3. Copy the `min` folder inside the [`monacoSRC`](/src/modules/previewpane/MonacoPreviewHandler/monacoSRC) folder.
+4. Generate the JSON file (see section below)
+
+## languages.json
+
+[`languages.json`](/src/modules/previewpane/MonacoPreviewHandler/languages.json) contains all extensions and Id's for the supported languages of Monaco. The [`FileHandler`](/src/modules/previewpane/MonacoPreviewHandler/FileHandler.cs) class and the installer are using this file.
+
+### Generate languages.json file
+
+After you updated monaco editor or adding a new language you should update the [`languages.json`](/src/modules/previewpane/MonacoPreviewHandler/languages.json) file.
+
+1. Build monaco in debug mode.
+2. Open [generateLanguagesJson.html](/src/modules/previewpane/MonacoPreviewHandler/generateLanguagesJson.html) in a browser.
+3. Replace the old JSON file.