docs(site-builder): add 5-article Site Builder section (Beta)

Adds /whp/site-builder/ with overview, getting-started, blocks-and-pages,
styling, and publishing. Wired as a 'Site Builder' sidebar group with a
Beta badge.

- Captured real screenshots via the demo account through a redaction
  step (server names, domain, demo-user all swapped for placeholders)
- New beta-callout partial shared across all 5 articles
- capture-site-builder.ts is local-only (uses tools/screenshots/.env
  for demo creds, never runs in CI)

src/content/partials/beta-callout.mdx

@@ -0,0 +1,3 @@
+<div class="anhh-accent-callout">
+  **Beta feature.** Site Builder is in active development. Some controls are still being refined and templates are being expanded. We'd love your feedback — use the **Submit feedback** link in the Site Builder header.
+</div>