Document how to access internal hard disks

Tails can access local hard disks when an administrative password is set at boot time. That ought to be documented (and also what can be dangerous such action).

Feature Branch: doc/5636-accessHD

Related issues

Has duplicate #6854 (closed)
Blocks #5398 (closed)
Blocks #7143 (closed)
Blocks #8881 (closed)

Original created by @tails on 5636 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information