Add documentation pages with interactive architecture diagram

- /docs route with sidebar navigation and index page
- Architecture section: system overview with React Flow diagram, data flow, database schema
- API reference, configuration guide, and self-hosting docs for Go DNS server
- Feature planning document for future development
- Added docs link to landing page nav

package.json

@@ -9,6 +9,7 @@
     "lint": "eslint"
   },
   "dependencies": {
+    "@xyflow/react": "^12.10.1",
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
     "lucide-react": "^0.575.0",