docs: revamp landing page with four-pillar card layout (#2531)

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

docs/toc.yml

@@ -30,6 +30,12 @@
     - name: Workflows
       href: reference/workflows.md
 
+# Concepts
+- name: Concepts
+  items:
+    - name: What is SDD?
+      href: concepts/sdd.md
+
 # Development workflows
 - name: Development
   items: