Backup: fixes and doc improvements

[ukkopahis]

untested - don't merge

Improvements:

• backup & restore documentation: Fix the "coming soon" placeholders. Include all useful stuff from docs/Backups.md.old.
• refactor: simplify by inlining the small scripts from scripts/backup_restore (each of them is invoked only from a single place)

Fixes:

• only stop/start the stack for backup, don't down/up it (which recreates containers into a different state if docker-compose.yml was edited or new images were pulled)
• chown files using the correct group
• work regardless of the current directory when script is executed
• automatically re-invoke script using sudo: a) to ensure no backup permission problems accessing files and b) restore is done using the correct file owners and permissions.
• chown all of ./backup to the appropriate non-root user

Closes #566, Fixes #439, Fixes #293, Fixes #254, Fixes #352