Update our doc to the new Persistent Storage

See #17803.

• Mention the Persistent Storage somewhere in "Encryption and Privacy". People might look there for ways to encrypt their files while the Persistent Storage is the best option for that.
• Split "Creating" and "Configuring". I'm more and more inclined to believe that short pages for each topics are better than longer pages that cover more ground, for example when people do internal searches or are only interested in one of the topic.
• Add a warning about breaking security where it matters the most:
  • ~/.ssh/config
  • `~/.gnupg/gpg.conf
  • Consider adding a README at the root of the system partition and Persistent Storage
• Explain better the security of the deletion
• Test search terms that test participants would have searched for:
  • "configure Tails Spanish keyboard"
  • "Wi-Fi connection" or "Tor Wi-Fi connection"
• Use .file.rc as example for Dotfiles
• Consider removing /doc/first_steps/persistence/warnings
• Consider renaming the page to persistent_storage
• Remove note about #18379 (closed)