Document the `theme` crate (#18690)

This PR enables required documentation for the `theme` crate starts on
documenting it.

The end goal is to have all meaningful documentation in the crate filled
out – However I'm not sure that just adding `#![deny(missing_docs)]` to
the whole crate is the right approach.

I don't know that having 200+ "The color of the _ color" field docs is
useful however–In the short term I've excluded some of the modules that
contain structs with a ton of fields (`colors, `status`, etc.) until we
decide what the right solution here is.

Next steps are to clean up the crate, removing unused modules or those
with low usage in favor of other approaches.

Changes in this PR:
- Enable the `deny(missing_docs)` lint for the `theme` crate 
- Start documenting a subset of the crate.
- Enable `#![allow(missing_docs)]` for some modules.


Release Notes:

- N/A