Rewrite docs/index.md from a philosophy essay into a landing page
organized around four pillars: Spec-driven by default, Use any coding
agent, Make it your own, and Integrate into your organization.
- Add hero section with GitHub Spec Kit branding and CTA buttons
- Add 2x2 pillar card grid with GitHub Primer color accents
- Add community stats section (96K stars, 200+ contributors, etc.)
- Add navigation cards and footer install CTA
- Move SDD philosophy content to docs/concepts/sdd.md
- Add custom DocFX template overlay with card CSS and dark mode
- Set landing layout for index.md via fileMetadata
- Update toc.yml and docfx.json for new concepts section
Move the Community Friends section from the main README into a dedicated
docs page at docs/community/friends.md, following the same structure as
the Reference section.
- New: docs/community/friends.md with content from README
- Updated: docs/toc.yml with Community section and Friends entry
- Updated: docs/docfx.json to include community/*.md in content glob
- Updated: README.md to link to the new docs page instead of inline list
