feat(docs): added 404 page for the docs (#2450)

* feat(docs): added 404 page for the docs

* added metadata

apps/docs/app/[lang]/not-found.tsx

@@ -0,0 +1,23 @@
+import { DocsBody, DocsPage } from 'fumadocs-ui/page'
+
+export const metadata = {
+  title: 'Page Not Found',
+}
+
+export default function NotFound() {
+  return (
+    <DocsPage>
+      <DocsBody>
+        <div className='flex min-h-[60vh] flex-col items-center justify-center text-center'>
+          <h1 className='mb-4 bg-gradient-to-b from-[#8357FF] to-[#6F3DFA] bg-clip-text font-bold text-8xl text-transparent'>
+            404
+          </h1>
+          <h2 className='mb-2 font-semibold text-2xl text-foreground'>Page Not Found</h2>
+          <p className='text-muted-foreground'>
+            The page you're looking for doesn't exist or has been moved.
+          </p>
+        </div>
+      </DocsBody>
+    </DocsPage>
+  )
+}