add temporary 404 page

client/src/pages/404.tsx

@@ -0,0 +1,12 @@
+export default function NotFound() {}
+
+// Temporary. Move to app/ directory when it's supported
+
+export function getStaticProps() {
+  return {
+    redirect: {
+      destination: "/",
+      permanent: true
+    }
+  };
+}