Separate difference between backup and export, add a restore-page

[davralin]

As discussed here discussions/772.

I converted default backup-method to just being a sql-dump, and added a docker-compose example.

I also created a dedicated restore-page, that lists all steps needed (including a re-scan).

I also migrated the former "yaml backup" to a dedicated export-document, and adjusted links regarding that there.

I'm not entirely sure what to do with the old user-guide/advanced/backup.md-file - as there's nothing really advanced covered there, everything is either covered in the getting-started/advanced/backup.md, or in user-guide/advanced/export.md instead.

Thoughts?

[CLAassistant]

All committers have signed the CLA.

[graciousgrey]

Thank you :)