Summary

• Replace standalone Vite demo app with a Docusaurus-powered website that combines interactive demo and documentation (concepts, guides, migration notes,
  API reference via typedoc)
• Remove CircleCI config — CI now runs entirely via GitHub Actions
• Add Cloudflare Pages preview deployment workflow for PRs
• Update PR template with Conventional Commits format hint

Details

The previous apps/demo-app was a standalone Vite app used only to showcase the library. This PR replaces it with a full apps/website Docusaurus site that
includes:

• Interactive demo at /demo (ported from the old demo app with simplified code)
• Documentation covering core concepts (tree structure, tabs, updates, controlled vs uncontrolled), guides (theming, custom toolbar, persisting layout,
  zero state), and migration from v6
• Auto-generated API docs via docusaurus-plugin-typedoc
• Live code examples via @docusaurus/theme-live-codeblock

The deployment workflow deploys to GitHub Pages on push to master and provides per-PR preview deployments via Cloudflare Pages.

Test plan

• npm run build:lib succeeds
• npm run build:site succeeds
• Demo page renders and is interactive (drag, resize, tabs)
• Documentation pages render correctly
• PR preview deployment posts a comment with a working URL