docs: consistent overview URLs, README revamp, homepage card polish

- Rename user-docs/intro to user-docs/ for consistent section landing
  pages (matching admin-docs/ and developer-docs/)
- Add redirect from /user-docs/intro for existing bookmarks
- Revamp README with Documentation header and three section links,
  condense quick-links row
- Remove emojis from homepage cards and developer-docs overview
- Widen subtitle container to prevent line wrap on homepage
- Fix Contributor Guide link to developer-docs/

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/sidebars.js

@@ -27,7 +27,7 @@ const sidebars = {
     {
       type: 'doc',
       label: 'Overview',
-      id: 'intro',
+      id: 'index',
     },
     {
       type: 'doc',