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)

astro.config.mjs

@@ -45,6 +45,11 @@ export default defineConfig({
               label: 'How-to guides',
               items: [{ autogenerate: { directory: 'whp/how-to' } }],
             },
+            {
+              label: 'Site Builder',
+              badge: { text: 'Beta', variant: 'tip' },
+              items: [{ autogenerate: { directory: 'whp/site-builder' } }],
+            },
             {
               label: 'Reference',
               items: [{ autogenerate: { directory: 'whp/reference' } }],