chore: migrate from MkDocs + Material to Zensical

[TomerFi]

MkDocs 2.0 is incompatible with Material for MkDocs. The Material team has moved to Zensical as a replacement.

Currently pinned at older mkdocs versions so nothing is broken, but Material for MkDocs will only receive security patches until ~Nov 2026.

Zensical reads mkdocs.yml natively, so migration should be straightforward.

Ref: https://squidfunk.github.io/mkdocs-material/blog/2025/11/05/zensical/