storage: Promote use of Cockpit for advanced disk functions like RAID and paritioning

For FreedomBox to be a proper NAS, it needs to be able to handle multiple disks and their setup into RAID. Fortunately, Cockpit provides this functionality. However, it is hard for a user to discover that such functionality exists in Cockpit. When a user visits storage module, talk about how RAID and partitioning functions can be accessed via Cockpit and show a link to Cockpit app. Also update the description of cockpit app to talk about RAID and partitioning of disks.