Migrate documentation toolchain from MkDocs/Material to Zensical

[gabrielbmoro]

Migrates the project's documentation pipeline from MkDocs + Material for MkDocs to Zensical, as per ADR-0001.

Changes

• .github/workflows/documentation.yml: Replace mkdocs gh-deploy --force with zensical build + peaceiris/actions-gh-pages@v4 for GitHub Pages deployment. Remove Configure Git Credentials step (handled by the action). Update pip cache key to zensical-.
• doc/4-tooling.md: Update local dev instructions — install zensical==0.0.46 instead of mkdocs-material, use zensical serve instead of mkdocs serve.
• AGENTS.md: Update generator and theme references.

Notes

• mkdocs.yml is left untouched — Zensical reads it as-is.
• Zensical version is pinned (0.0.46) to avoid breakage from alpha releases.
• peaceiris/actions-gh-pages publishes ./site to the gh-pages branch.