Populating Production Gitlab Backup and Restore steps

General MR

Summary

Adding docs/guides/gitlab-backup-restore.md to have a location with recommendations for customers to follow to perform manual backups and set up automated backups for their Gitlab installations.

Preview: https://repo1.dso.mil/platform-one/big-bang/bigbang/-/blob/gitlab-backup-documentation/docs/guides/gitlab-backup-restore.md

Closes https://repo1.dso.mil/platform-one/big-bang/apps/developer-tools/gitlab/-/issues/146