Explain how to restore the configuration into new machine/Backrest instance

[tredondo]

Is your feature request related to a problem? Please describe.
I've backed up my config file as advised at https://garethgeorge.github.io/backrest/introduction/getting-started/, "Consider storing your entire Backrest configuration"

Now I want to restore that config on a new machine, but simply copying the backed up config file into ~/.config/backrest and reloading http://localhost:9898/ doesn't show my previous configuration.

Describe the solution you'd like
Both https://garethgeorge.github.io/backrest/introduction/getting-started/ and the /#/getting-started page should explain/prompt the user to restore the configuration.

Additional context
Not sure if sudo systemctl resrtart backrest.service then reloading the web UI is enough, because I copied the repos key from the backed up config into a new blank config.json generated from the web UI (different sync, instance, and modno keys).

Related: #1012

[Alk2]

On Windows, I just successfully tested a restore from a backed up version of the config.json file in the %APPDATA% eg: C:\Users%username%\AppData\Roaming\backrest folder (backrest continually makes backups every time you make a config change). After restoring the file, I had to Quit Backrest from the system tray and re-open it from c:\program files\backrest\backrest.exe for the changes to take effect.