Migrate site from Docusaurus to Astro

[tgrall]

Why

The current Docusaurus 2 site is difficult to maintain and no longer builds reliably on current Node.js releases. This migration moves the site to Astro 7 while preserving the published content contract and modernizing the design and deployment workflow.

What changed

• Rebuilt the static site with Astro 7, scoped CSS design tokens, responsive layouts, and persisted light/dark themes.
• Preserved all 275 article URLs, publication metadata, blog pagination, archive pages, 105 tag routes, redirects, sitemap behavior, and 275-entry RSS and Atom feeds.
• Vendored 20 public Gists so historical articles no longer depend on runtime third-party embeds.
• Reimplemented the homepage, talks, Copilot, Quarkus, showcase, and 404 experiences.
• Replaced the legacy deployment workflow with the official GitHub Pages artifact flow and added scheduled production drift and external-link checks.
• Removed Docusaurus, React-specific site components, starter documentation, and Yarn artifacts.

Validation

• Deterministic comparison against a committed production baseline covering 426 published routes, normalized article content, metadata, redirects, internal links, fragments, local assets, and feeds.
• 4 unit tests for route, date, tag, and content compatibility behavior.
• 28 Playwright checks across desktop and mobile, including accessibility, theme persistence, redirects, showcase behavior, and visual snapshots.
• Clean npm install, Astro type check/build, production parity validation, and dependency audit all pass.

Review notes

The only explicit semantic-content allowances are seven articles whose remote Gists are now rendered from vendored local data and the 2026 Copilot CLI article whose interactive component was intentionally redesigned. Feed IDs and links remain byte-for-byte stable so existing readers do not see historical entries as new items.