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
