53 integate mermaidjs rendering with pozeiden

[sc2ben]

Summary

• Add Mermaid.js rendering to HTML and Typst via pozeiden (lazy dep; falls back to plain code block on error or when unavailable); add src/noop_mermaid.zig stub for builds without pozeiden
• Add nix build .#site combining the WASM playground and API docs for zigmark.sc2.in; add Docs/GitHub nav links to the playground header; trim sources.tar 35 MB → ~500 KB by repacking with only project sources
• Fix update-zon: drop zig fetch --save . which caused a self-referential hash loop; apply env -u ZIG_GLOBAL_CACHE_DIR consistently to all zig2nix zon2lock calls
• Add .github/workflows/deploy-docs.yml to build and deploy the site to Cloudflare Pages on push to main

Test plan

• zig build test passes (unit tests + CommonMark + GFM spec)
• Mermaid stub tests pass for HTML and Typst (error and null-renderer fallback cases)
• update-zon no longer corrupts build.zig.zon with a self-dep
• nix build succeeds with updated lock file
• nix build .#site succeeds; result/docs/sources.tar under 25 MB
• Playground loads; Docs and GitHub links visible in header
• /docs renders without panic
• mermaid diagrams render on playground