Add API and docs for System Backups

Pterodactyl Panel have API to get list of backups and the detail (

https://pterodactyl-api-docs.netvpx.com/docs/api/client/backups#list-backups). In UltraServers, this is the “User Backups” tab, and not “System Backups”. Hopefully there would be similar API for the System Backups, as well as the docs.