docs(readme): consolidate Community Friends sections and fix ToC anchors (#1958)

* docs(readme): consolidate Community Friends sections and fix ToC anchors

- Merge duplicate 🤝 Community Friends section (table format near bottom) into
  the existing 🛠️ Community Friends section (bullet list)
- Add cc-sdd entry alongside Spec Kit Assistant
- Update intro text to 'Community projects that extend, visualize, or build on Spec Kit'
- Fix ToC anchors for Video Overview and Community Friends (remove variation selector from fragment)

* docs(readme): remove stale ToC entry for deleted Community Friends section