Switch from Material for MkDocs to Zensical

[itzg]

Material for MkDocs is now in maintenance mode.

The team is focusing on
https://squidfunk.github.io/mkdocs-material/blog/2025/11/05/zensical/

Read the docs already supports it
https://docs.readthedocs.com/platform/stable/intro/doctools.html

[EmilyxFox]

I'm taking a look at this 🫡

[itzg]

Completed in #3852