Better docs about Backup / Restore

I think the project is missing documentation on how to properly create backups and restore if quorum is lost or if some unit has a corrupted database.

I can see that there is the `CreateDatabaseBackup` and `RecoverFromQuorumLoss` methods in the code base, however I don't see an easy on how to use on projects that uses microcluster like sunbeam.

I've tested copying the `state-dir` content, pausing units and restoring the content works fine, but would be nice to have a easy and automate way of doing in projects.