Document GNOME Disks for Debian, Linux, and manual upgrades

Remove “intermediary” Tails from Linux steps
- Reuse GNOME Disks instructions from Linux in Debian steps
- Replace Tails Installer by GNOME Disks to install intermediary Tails in manual upgrade steps
- Is it better to keep Debian and Linux separated?

Worker: Cody, reviewer: sajolida (1h)

Original created by @u on 15659 (Redmine)

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