Add manual pages and content for upcoming GDScript trait system

The trait system for GDScript being worked on at https://github.com/godotengine/godot/pull/97657 appears to be mostly done, but "Write Documentation" is one of the items still unchecked. The main Godot repository only contains the class reference, which isn't especially applicable to a new language feature.

I think it'll be important for the success of this feature to prepare the user manual to describe traits, so that as soon as they're available in a stable release, users can read about them and understand how to use them.

*Edit: A PR has been opened at https://github.com/godotengine/godot-docs/pull/10393, and progress on completing the necessary work is now being documented there!*