feat: separate current and "new" web in route groups

app/[lang]/(preview-current)/not-found.tsx

@@ -0,0 +1,8 @@
+export default function NotFound() {
+  return (
+    <main>
+      <h2>Not Found</h2>
+      <p>Could not find requested resource</p>
+    </main>
+  )
+}