Missing proper GDExtension docs

**Your Godot version:**
4.6
**Issue description:**
C++ GDExtension is not well-documented.
- There's no explaination on the GDExtension docs on how to use `Ref<>` properly
- There's no explaination on the GDExtension docs what does `memnew` macro do and where to use it
- There's no explaination on the GDExtension docs on what does `instantiate` here, and how to use it properly

If this is documented elsewhere, please add links in a visible place to GDExtension docs, I think it might be really helpful
**URL to the documentation page (if already existing):**