[pyrech]

Same migration as jolicode/castor#918: Zensical is the successor of Material for MkDocs by the same team. The build stays in Docker (no cairo needed any more for the social cards), tools/mkdocs/ becomes tools/doc/ and the include-markdown directives become markdown() macros (tools/doc/main.py).

Most pages show Twig syntax (the Dockerfile templates), so the macros are opt-in: only index.md and changelog.md declare render_macros: true.

Output checked against a MkDocs build of the same sources: same 30 pages and social cards, same content, headings, admonitions and code blocks. The relative links of the included CHANGELOG.md are now resolved to pages (they were left as .md links before). The modern theme variant brings the JoliCode colors from oss-theme/Zensical/extra.css. Only visible changes: the Zensical search widget, the "On this page" wording and the section index no longer repeated at the top of the sidebar.

Also in separate commits: two links to the ghcr.io section used a wrong anchor (the strict build catches it now), and a docs CI job building the documentation.

[joelwurtz]

Thanks