Update and improve Azure and LVM snapshot backups documentation

Overview

Continuing: https://gitlab.com/gitlab-com/infrastructure/issues/28

Our backup documentation needs an overall:

- Move backup.md to runbooks
- Include information for recovering from (local host) LVM snapshots
- Remove outdated ebs information
- Document possible issues/troubleshooting when restoring Azure snapshots (lessons learnt in https://gitlab.com/gitlab-com/infrastructure/issues/252 )