Publish Markdown documentation with VitePress

[cazala]

Summary

• generate a static VitePress documentation site at /party/docs/ from the existing Markdown guides
• use README.md as the documentation homepage source and rewrite its repository-relative links only in generated HTML
• provide global Documentation, Playground, and GitHub navigation plus local search, unique metadata, canonical URLs, and a sitemap
• point the playground Learn button to the generated docs
• merge the docs build into the existing Cloudflare Pages artifact without changing the visual homepage
• preserve both origin-style and /party/-prefixed shared-session SPA fallbacks

Implementation notes

• VitePress builds after the playground because Vite recreates packages/playground/dist
• guide source-code links are rewritten to their corresponding GitHub files during rendering
• the broad Pages catch-all rewrite was removed so real /docs/* assets can be served; Pages default SPA fallback continues handling session paths

Verification

• npm run build
• npm run type-check
• npm run build:docs
• pnpm install --frozen-lockfile --offline
• git diff --check
• local wrangler pages dev checks for docs home, all guide routes, docs assets, preview-prefixed routes, and shared-session fallback routes

[github-actions]

Preview deployment ready

• URL: https://ac6233db.party-playground.pages.dev
• Commit: b754684