Add trait tag for items

We can use the same approach from bevy https://github.com/bevyengine/bevy/tree/release-0.16.0/docs-rs.

Providing the Animation Traits should be useful for doc, e.g., Interpolatable, Alignable, Stroke, Fill, ...

Start with the linked Bevy release-0.16.0/docs-rs tree as the reference for trait tags. Determine which ranim entry point controls the documentation for Animation Traits such as Interpolatable, Alignable, Stroke, and Fill, then verify that generated documentation displays those tags for the relevant items.

Written by the indexing model from the issue text.