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)
