Documentation on changing root paths in config

[patrickwolf]

Is your feature request related to a problem? Please describe. In windows when i plug in a usb drive i might get a different drive letter. backrest does not allow me to change the path in the UI. I need to create a new repo and a new plan!

Describe the solution you'd like Add documentation where the config files are stored per platform or how to find it in the UI. Let users know they can update it while backrest is closed to get around the problem

Additional context Its a simple solution that gets people going again ;)