Document backup/restore between Helm- and Operator-based installations

Summary

Related to #312 (closed)

We should test the backup and restore process for the following flows:

• Backup in Helm-based installation and restore in Operator-based installation
• Backup in Operator-based installation and restore in Helm-based installation

Acceptance criteria

• Backup and restore process has been tested in both directions as outlined above
• Any information that is not documented in https://docs.gitlab.com/charts/backup-restore/ is added to the Operator documentation