fix(app-router): remove pages 404/_document; add app/not-found

src/app/not-found.tsx

@@ -0,0 +1,8 @@
+export default function NotFound() {
+    return (
+        <main style={{ padding: 24 }}>
+            <h1>Not Found</h1>
+            <p>Sorry, the page you are looking for does not exist.</p>
+        </main>
+    );
+}