chore: migrate from MkDocs + Material to Zensical #821
Description
MkDocs 2.0 is incompatible with Material for MkDocs. The Material team has moved to Zensical as a replacement.
Currently pinned at mkdocs==1.6.1 so nothing is broken, but the warning appears in CI (Stage workflow). Material for MkDocs will 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/