docs: Docusaurus documentation site + GitHub Pages workflow

[NikPnevmatikos]

What this adds

• website/: a Docusaurus 3.10 site with eight pages moved from the README (getting started, supported HTML, styling and cascade, props, custom rendering, plugins, how it works, limitations). The landing page embeds example/App.tsx in Expo Snack on SDK 56 as a live playground. The folder is deliberately not an npm workspace member; it has its own package.json and lockfile.
• .github/workflows/docs.yml: builds the site on pull requests that touch website/ (this PR will run it as a check), and builds + deploys to GitHub Pages on push to main.
• README badge (root and packages/core) linking to https://nikpnevmatikos.github.io/Html-Renderer/.

Verified locally

• npm run build in website/ succeeds with onBrokenLinks and onBrokenAnchors set to throw.
• npm run typecheck in website/ passes.
• Served the build and confirmed the Snack embed creates its iframe with sdkVersion=56.0.0 and platform=web, and that the docs pages render with sidebar and table of contents.

Before merging: enable GitHub Pages (one time)

Repository Settings → Pages → Build and deployment → Source: GitHub Actions. Nothing else to configure. Until this is done the deploy job fails on the first push to main, and the README docs badge 404s.

Notes

• The README still documents everything in full. Trimming it to install + quick start + links is a possible follow-up, left out here to keep the review focused.
• The README's "Current limitations" says cell width is ignored; the changelog for 0.4.0 says it is honored. The new Limitations page follows the changelog. Worth aligning the README in a follow-up.

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