[ACA-1889] Split Extending section into separate files (#784)

* [ACA-1889] Split Extending section into separate files

* [ACA-1889] Split Extending section into separate files

* [ACA-1889] Added recent changes and then deleted extending.md

* [ACA-1889] Added Tutorials section

docs/extending/extensibility-features.md

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+---
+---
+
+# Extensibility features
+
+Application extensibility is performed via the root `/src/assets/app.extensions.json`,
+and any number of external plugins that are references of the main entry point.
+
+The application also comes with the `/src/assets/plugins/` folder
+already preconfigured to store external files.
+
+You can create plugins that change, toggle, or extend the following areas:
+
+- Navigation sidebar links and groups
+- Context Menu
+- Sidebar (aka Info Drawer)
+- Toolbar entries
+  - buttons
+  - menu buttons
+  - separators
+- Viewer actions
+  - "Open With" entries
+  - toolbar entries
+    - buttons
+    - "More actions" buttons
+- Content metadata presets (used on `Properties` tab)
+
+Extensions can also:
+
+- Overwrite or disable extension points of the main application or other plugins
+- Change rules, actions or any visual element
+- Register new application routes based on empty pages or layouts
+- Register new rule evaluators, components, guards