Add authentication information for S3 and Ceph to the Backup and Restore doc

[hallaroo]

Currently the 'Authentication' section only references information on Google Cloud Storage plugin and not the other two plugin options (S3 and Ceph).

We should add information about how to authenticate for the other two options.

[AnthonyPoschen]

Just installed the operator and this was the first thing i wanted to know the requirements for S3 to function.