feat: changelog content type with RSS feed

[DanFlannel]

Summary

Adds a dated changelog/release-notes content type — the last feature gap vs
Docusaurus. Design principle: entries ride the existing docs machinery rather
than a separate blog engine, so search, prerendering, OG cards, edit links,
and hydration all work with zero new plumbing.

Changelog — drop dated MDX files in content/changelog/:

• Changelog tab (href-only, like API Reference) + newest-first listing at
  /docs/changelog with dated cards (title, date, description).
• Entries are ordinary docs at /docs/changelog/<slug> — searchable,
  prerendered with OG cards, edit-on-GitHub linked — with the entry date shown
  in the breadcrumb. They stay out of the Documentation sidebar and the docs
  index (their tab owns discovery).
• date frontmatter (ISO YYYY-MM-DD, quoted or unquoted — gray-matter's
  parsed Date objects are normalized) flows through the manifest.
• RSS 2.0 feed at /feed.xml on deploys with VITE_SITE_URL set, newest
  first with RFC-1123 pubDates and absolute links; every page head advertises
  it via <link rel="alternate">. Cleanly absent (file and tag) without a
  site URL — same gating as the sitemap.
• The listing prerenders through the existing pageHtml() with a synthetic
  manifest entry, so it gets a real title/canonical and a "Changelog" OG card
  for free.
• Date rendering is pure string math ("Jul 10, 2026") — never
  Date/toLocaleDateString, whose UTC-midnight parsing shifts a day across
  timezones and would break hydration of prerendered pages.

Also in this PR:

• Doc pages now prefer the frontmatter title over the filename-derived one in
  the breadcrumb and <title>/og:title ("v0.2.0 — …" instead of "V0 2 0").
• Two real demo entries (v0.1.0, v0.2.0) dogfooding the feature, plus a
  Guides → Changelog authoring guide.
• Fixed a stale "five built-in styles" → four in the introduction.

Type of change

• New feature
• Docs / content

Checklist

• pnpm typecheck passes
• pnpm lint passes
• pnpm format:check passes (run pnpm format to fix)
• pnpm build succeeds
• pnpm check:links passes
• Docs/examples updated if behavior changed

Notes

• e2e grew 21 → 25 tests: prerendered listing order, entry hydration (guards
  the TZ-free date rendering), sidebar/docs-index exclusion, and Pagefind
  discoverability of entries.
• feed.xml is a generated artifact (gitignored, like the sitemap).
• To hide the feature entirely: delete content/changelog/ and remove the tab
  from src/navigation.ts.

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