Centralize theme documentation

[kimisgold]

The theme-docs branch has a section for documenting themes and their settings as part of an initiative to centralize that reference material. They're largely mirroring what's been provided on the theme READMEs. Here are the main points I see for review and discussion:

• Coverage - Are any major gaps in what the docs currently touch on?
• Style - Do these entries feel consistent with the rest of the user manual?
• Up-to-date - This branch was created at the beginning of 2025; where does this branch need to be updated?