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

app/page.tsx

@@ -1,3 +1,4 @@
+import Link from "next/link";
 import { Globe, Search, Shield, Activity } from "lucide-react";
 import {
   Card,
@@ -42,7 +43,15 @@ export default function Home() {
               VectorDNS
             </span>
           </div>
-          <ThemeToggle />
+          <div className="flex items-center gap-4">
+            <Link
+              href="/docs"
+              className="text-sm text-muted-foreground transition-colors hover:text-foreground"
+            >
+              Docs
+            </Link>
+            <ThemeToggle />
+          </div>
         </div>
       </header>