Migrate to app router (#2254)

app/not-found.tsx

@@ -0,0 +1,29 @@
+import Link from "../components/Link";
+import Video from "../components/Video";
+import type { Metadata } from "next";
+
+export const metadata: Metadata = {
+  title: "404 Not Found",
+};
+
+export default async function Page() {
+  return (
+    <div style={{ textAlign: "center" }}>
+      <Video
+        src={{
+          webm: "/static/images/angry-panda.webm",
+          mp4: "/static/images/angry-panda.mp4",
+        }}
+        autoplay
+        responsive={false}
+        style={{
+          maxWidth: "400px",
+        }}
+      />
+
+      <h1>404: Page Not Found 😢</h1>
+
+      <Link href="/">Go home?</Link>
+    </div>
+  );
+}