Generate Godot user documentation from kdoc

[chippmann]

With Godot 4, one can now provide documentation for all custom classes and members.

Note: After we moved to GDExtensions, the following info is not correct anymore. However even though the api will change, fundamentally the concept will be the same.

We can provide the same now as the script class (and thus also our kotlin_script class provide a virtual function get_documentation which lets us provide documentation for the classe and it's members.

Now we could extract the kdoc from the users code and provide that through the entry gen to godot so it can be displayed in the editor.

[CedNaru]

Don't start implementing that for scripts, this change will be irrelevant when we switch to extensions.
Extensions do provide a way to set documentation of course, but it's a different API.

[chippmann]

True, still we can implement that once we're there