Document intended scope for GDExtension: What is expected to be accessible to GDExtension developers?

**Your Godot version:**
4.1

**Issue description:**

Make GDExtension documentation accessible to 3rd party developers looking to integrate their tools, SDKs, etc into Godot. This means clear information on export platform support (any special considerations for mobile or web?), and clear information on where exactly GDExtension does not go as deep as engine modules. In this case, AnimationPlayer is the relevant discussion point. It's accessible via engine module route, but can it also be extended via GDExtension? A 3rd party software developer that supports a couple dozen integrations for various engines and frameworks doesn't have the time to tinker and experiment. This is where ideally docs would come in and bridge the knowledge gap. Since the docs mention engine modules having deeper engine integration, it gets chosen by default, which is not ideal.

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

https://docs.godotengine.org/en/stable/tutorials/scripting/gdextension/what_is_gdextension.html