Add a sample of GDExtention's `dependencies` syntax

**Your Godot version: ** 4.2.2

Issue description: dependencies is mentioned in the docs here, but there is no example here of what that syntax looks like.

URL to the documentation page (if already existing): https://docs.godotengine.org/en/4.0/tutorials/scripting/gdextension/gdextension_cpp_example.html#using-the-gdextension-module

PR https://github.com/godotengine/godot-docs/pull/9383 is attempting to add more documentation about .gdextension files, but it doesn't cover dependencies either