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/docfx.json

@@ -6,6 +6,7 @@
           "*.md",
           "toc.yml",
           "community/*.md",
+          "concepts/*.md",
           "reference/*.md",
           "install/*.md"
         ]
@@ -50,7 +51,8 @@
     "fileMetadataFiles": [],
     "template": [
       "default",
-      "modern"
+      "modern",
+      "template"
     ],
     "postProcessors": [],
     "markdownEngineName": "markdig",
@@ -68,6 +70,11 @@
         "repo": "https://github.com/github/spec-kit",
         "branch": "main"
       }
+    },
+    "fileMetadata": {
+      "_layout": {
+        "index.md": "landing"
+      }
     }
   }
 }