Evaluate migrating to Zensical from material+mkdocs

The material for mkdocs theme is no longer under active development. Read why here https://squidfunk.github.io/mkdocs-material/blog/2025/11/05/zensical/#why-zensical

It looks like they are maintaining some amount of backwards compatibility and integrating some of the more popular plugin functionality.

I think the main things we'll to check are
- Versioning - Can we work in the same way as we do with `mike?` Can we carry over our old "built" docs on our `gh-pages` branch to this new solution?
- Plugins - Are we relying on plugin functionality that hasn't been reimplemented in Zensical?

https://zensical.org/

Start with the linked Zensical rationale and the project's current mike and plugin setup. Check whether versioning and existing built documentation on the gh-pages branch can be carried over, then compare each required plugin with Zensical's supported functionality. Done means a documented migration assessment, including compatibility gaps and open questions.

Written by the indexing model from the issue text.