Adjust the documentation wrt. updated system partition size

See the parent ticket for details. I think the only needed change is replacing “4 GiB” with “8 GiB” (modulo units) in various places, as all the special cases will be handled by messages in Tails Installer (and perhaps Upgrader too) but please double-check :)

Feature Branch: devel

Parent Task: #12705 (closed)

Related issues

Related to #14624 (closed)
Blocks #13234 (closed)
Blocks #13423 (closed)

Original created by @intrigeri on 12706 (Redmine)

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