Change the custom dock tutorial to recommend EditorDock as scene root

**Your Godot version:**

4.7

**Issue description:**

> For an editor dock, the root node must be a [Control](https://docs.godotengine.org/en/latest/classes/class_control.html#class-control) or one of its child classes.

This is a bit outdated. It's better to use EditorDock as a scene root of a dock, which also allows to set up its properties in the inspector.
Note that https://github.com/godotengine/godot/pull/111518 should be merged first to allow adding EditorDock easily.

**URL to the documentation page:**

https://docs.godotengine.org/en/latest/tutorials/plugins/editor/making_plugins.html#a-custom-dock