add missing _index.md file
add missing _index.md file

Change-Id: Id3c3ca0d143e9fc76f475e42789bdf5e2b35f1f8
GitOrigin-RevId: e53bd0d068124b7c3518708c4aef0c7b0a968512

GoogleCloudPlatform/gcpdiag,GoogleCloudPlatform/gcpdiag,GoogleCloudPlatform/gcpdiag