Add documentation for Using a Type Variation example in code

[tom-con]

Your Godot version:
4.5.1
Issue description:
Currently the documentation only has implementation details for Using a Type Variation via the interface; it would be useful to have a coding example.
URL to the documentation page (if already existing):
https://docs.godotengine.org/en/stable/tutorials/ui/gui_theme_type_variations.html#using-a-type-variation