docs(antora): switch Mermaid to offline rendering via "markup-antora"…

[michalharakal]

… image

Migrate the docs image to the consolidated "markup-antora" definition (one image shared across the SKaiNET docs projects, vendored here until the public registry image is published) and switch Mermaid rendering from the external Kroki round-trip to fully offline.

Playbook: drop asciidoctor-kroki + kroki-fetch-diagram in favour of /opt/antora/local-mermaid-extension.js; add runtime.cache_dir so the container runs rootless. Matches mainline SKaiNET.

Features of the merged image:

• Offline Mermaid: every [mermaid] block is rendered to inline SVG at build time by mermaid-cli (Alpine Chromium + Puppeteer). No Kroki, no kroki.io, no network at build or view time. Removes the asciidoctor- kroki 4 KB GET-URL limit that rejected large diagrams.
• Diagram caching: content-hash, in-memory + optional on-disk (MERMAID_CACHE_DIR); identical diagrams render once.
• Rootless-safe under --user $(id -u):$(id -g) (HOME=/tmp + cleanup of root-owned /tmp dirs; fixes Chromium crashpad / cosmiconfig EACCES).
• Build-time mermaid smoke test (a broken image fails docker build).
• Offline extras available: @antora/lunr-extension, pre-baked Antora UI bundle, MathJax es5; asciidoctor-kroki kept as an escape hatch.
• Full Alpine font set for diagram labels, emoji and CJK.

CI is unchanged: docs.yml already builds from docs/.docker.

Usage and details: see docs/.docker/README.md.