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