chore: migrate docs site from Material for MkDocs to Zensical

[dgarros]

Overview

Migrate the docs site from Material for MkDocs to Zensical, the successor project from the same team.

Motivation

Material for MkDocs is entering maintenance mode — the team will only address critical bugs and security issues for ~12 months while active development moves to Zensical. MkDocs itself has been unmaintained since August 2024. Zensical is designed as a drop-in successor: it reads existing mkdocs.yml and Markdown files natively, and delivers 4–5x faster rebuilds.

See the announcement: https://squidfunk.github.io/mkdocs-material/blog/2025/11/05/zensical/

Acceptance Criteria

• Project builds and serves locally using Zensical instead of mkdocs / mkdocs-material.
• Existing mkdocs.yml, Markdown content, and any theme overrides render correctly.
• pyproject.toml dependencies updated (remove mkdocs, mkdocs-material; add Zensical).
• .github/workflows/docs.yaml updated to build and publish via Zensical.
• README.md and CONTRIBUTING.md updated with the new commands.

Context

• Not urgent — Material for MkDocs will keep working for at least 12 months.
• Worth waiting until Zensical matures further (module system, component system, CommonMark support are still rolling out) before migrating.

References

• Zensical announcement: https://squidfunk.github.io/mkdocs-material/blog/2025/11/05/zensical/
• Zensical: https://zensical.org/
• Related: chore: convert project from Poetry to uv #8 (uv migration)