Add class reference documentation for GDExtension & GDExtensionManager

[Mickeon]

This PR fills in the class reference of GDExtension and GDExtensionManager. It also links to the existing tutorials.

I don't know what else to say.

More feedback and useful insight is necessary from actual GDExtension developers. A few of these descriptions are unfortunately still barren.

[dsnopek]

Thanks for being the first person brave enough to attempt to document these classes :-)

[dsnopek]

Maybe:

Suggested change

	Initializes the library bound to this GDextension.
	Initializes this GDExtension at the given initialization level.

This should probably also have a note saying that developers shouldn't call this under normal circumstance, it will be handled by GDExtensionManager.

I'm actually not entirely sure why we've exposed this at all? It's especially weird considering we haven't exposed deinitialize_library(), its paired method.

[Mickeon]

I'm actually not entirely sure why we've exposed this at all? It's especially weird considering we haven't exposed deinitialize_library(), its paired method.

It may be worth discussing this with the rest of the GDExtension team. There have been times before where Reduz and/or others accidentally exposed methods to the documentation & scripting language "just in case" (hence the name of the PR).
After a lot of time has passed, only then you can tell there's a flaw in the API when struggling to write documentation for these.

[dsnopek]

I've added an item to the agenda of tonight's GDExtension meeting, to discuss if this method should even be bound in the first place

[Mickeon]

Addressed all of the above feedback and suggestions. Thank you very much for the insight

[AThousandShips]

Looks good otherwise

[Mickeon]

Thank you, I'm very happy to have this as soon as possible.

[dsnopek]

Thanks!

While I personally think we should probably unbind GDExtension::initialize_library(), and hence don't need docs for it, that doesn't necessarily need to hold up this PR.

This looks great!

[Mickeon]

Accepted the above suggestion

[akien-mga]

Thanks!