[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/actions.md

@@ -0,0 +1,55 @@
+---
+---
+
+# Actions
+
+| Name | Description |
+| -- | -- |
+| **id** | Unique identifier. |
+| **type** | Action type, see [Application Actions](/extending/application-actions) for more details. |
+| **payload** | Action payload, a string containing value or expression. |
+
+```json
+{
+  "$schema": "../../../extension.schema.json",
+  "$version": "1.0.0",
+  "$name": "plugin1",
+
+  "actions": [
+    {
+      "id": "plugin1.actions.settings",
+      "type": "NAVIGATE_URL",
+      "payload": "/settings"
+    },
+    {
+      "id": "plugin1.actions.info",
+      "type": "SNACKBAR_INFO",
+      "payload": "I'm a nice little popup raised by extension."
+    },
+    {
+      "id": "plugin1.actions.node-name",
+      "type": "SNACKBAR_INFO",
+      "payload": "$('Action for ' + context.selection.first.entry.name)"
+    }
+  ]
+}
+```
+
+## Value expressions
+
+You can use light-weight expression syntax to provide custom parameters for the action payloads.
+
+```text
+$(<expression>)
+```
+
+Expressions are valid JavaScript blocks that evaluate to values.
+
+Examples:
+
+```text
+$('hello world')                //  'hello world'
+$('hello' + ', ' + 'world')     //  'hello, world'
+$(1 + 1)                        //  2
+$([1, 2, 1 + 2])                //  [1, 2, 3]
+```