Slightly better not-found page

docs/app/not-found.tsx

@@ -1,5 +1,18 @@
 import * as React from 'react';
 
+import { AppBar } from 'docs-base/src/components/AppBar';
+import { Navigation } from 'docs-base/src/components/Navigation';
+import routes from 'docs-base/data/pages';
+import classes from './(content)/styles.module.css';
+
 export default function NotFound() {
-  return <h1>Page not found</h1>;
+  return (
+    <React.Fragment>
+      <AppBar />
+      <Navigation routes={routes} />
+      <main className={classes.content}>
+        <h1>Page not found</h1>
+      </main>
+    </React.Fragment>
+  );
 }