Add clear instructions on how an end user can use a pre-compiled GDExtension in their project

[muno777]

Your Godot version: 4.2

Issue description: The current documentation for GDExtension seems to focus on the developer experience, without much to offer for users who wish to use an extension in their own project by downloading a .gdextension file. This threw me for a loop initially until I realised that it's as simple as adding the file to the project folder (unlike addons, which need to be enabled in the project settings). To help others learn this more easily, it would be nice if the documentation included a brief section addressed to end users.

URL to the documentation page (if already existing): Link