Add copyright header for MDXComponents.js (#3083)

Author: zhenbang-2026

website/src/theme/MDXComponents.js

@@ -1,3 +1,9 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
 // Import the original mapper
 import MDXComponents from '@theme-original/MDXComponents';
 import GithubLink from '@site/src/components/GithubLink';