Implement missing descriptions of Persistent Storage features

Leftover from #17803 (closed).

The current implementation doesn't include the description / hint / subtitle below some of the Persistent Storage features.

From https://gitlab.tails.boum.org/tails/blueprints/-/wikis/Persistent-Storage-redesign:

• Persistent Folder
  • Create a folder where you can save your documents
• Network Connections
  • Save Wi-Fi passwords and mobile broadband configuration
• GnuPG
  • Save OpenPGP keys outside of Thunderbird, like Kleopatra
• Additional Settings
  • Install additional software automatically when starting Tails
• Dotfiles
  • Symlink every file in the Dotfiles folder into the Home folder

User research findings

• During #18648 (closed), 3 out of 4 participants got mildly confused about what the "Carpeta Persistente" was about and I think that a subtitle would have helped, as part of basic onboarding to the Persistent Storage:
  • P1 thought that turning off the Persistent Folder would delete everything, including browser history.
  • P4 wondered what to turned on to save an image to the USB stick.
  • P3 and P4 didn't understand at first what opens from the shortcut to the Persistent Folder