docs: switch to zensical

[zowoq]

https://squidfunk.github.io/mkdocs-material/blog/2025/11/11/insiders-now-free-for-everyone/
As we're shifting our efforts to Zensical, Material for MkDocs is entering maintenance mode. This means that while we'll continue to fix critical bugs and security issues for 12 month at least, no new features will be added to Material for MkDocs.