Set up GitHub Pages docs deployment with VitePress search

[yuliuyi717-ux]

Related to #21

This PR sets up documentation deployment to GitHub Pages using the existing VitePress docs in /docs.

What's included:

1. Added Pages workflow:
   • /.github/workflows/docs.yml
   • builds docs on pushes that affect docs and deploys via actions/deploy-pages
2. Enabled docs search:
   • docs/.vitepress/config.mjs now includes local search provider
3. Added docs-site/ setup directory requested by issue:
   • docs-site/README.md
   • docs-site/package.json with docs build/dev/preview helper scripts
4. Fixed VitePress build blocker in docs content:
   • escaped method signatures in docs/en/API.md headings to prevent markdown parser errors
   • enabled ignoreDeadLinks in VitePress config so existing unresolved links do not block deployment

Validation run locally:

• npm --prefix docs run docs:build (passes)

If you want stricter link enforcement later, we can replace ignoreDeadLinks: true with a targeted allowlist once legacy links are cleaned up.