Shift from Material for MkDocs to Zensical

[halostatue]

This is mostly a tracking issue since Zensical was announced a few days ago and they have put Material for MkDocs into maintenance mode as they are spending their time on Zensical.

There's a few plugins being used (I added a couple) which may or may not be compatible, so we should plan a migration.

I'll add more to this when I know more.

[twpayne]

Thank you for tracking this!