Add GCP to the Back up large environments doc

The following discussion from !125182 (merged) should be addressed:

• @sranasinghe started a discussion: (+3 comments)

  @mkozono how about including GCP options here as well? We can have self-contained instructions for AWS and GCP.

  We should probably state that we recommend using AWS/GCP tools to backup DB if it is managed and allude to the alternative option if it is not.