Write documentation for runtime classes

Runtime classes were added in Godot 4.3, and can be used both from GDExtension and modules (although, there's no reason to use from a module unless you are converting a GDExtension to a module), but we don't have any documentation for them yet.

Should cover `GDREGISTER_RUNTIME_CLASS()` and cover what that actually means for the class.

A common issue folks hit with godot-cpp, is attempting to do `Object::cast_to()` to the class in an editor plugin, and being confused as to why they get `nullptr`, so that should be explained as well. (The reason is that runtime classes aren't created in the editor by default, instead the native parent class is created, so you can't cast to the real class.)

Creating this issue so contributors can take on this task if they'd like :-)