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:
+ [x] - Move [backup.md](https://dev.gitlab.org/cookbooks/chef-repo/blob/master/doc/backups.md) to [runbooks](https://gitlab.com/gitlab-com/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+ [ ] - Remove outdated ebs information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 )
//cc @marat
issue