Explain better the process when automatic upgrades don't work anymore

Originally created by @BitingBird on #9972 (Redmine)

Looks like lots of people arrive to the moment where they can’t automatically upgrade anymore. https://tails.boum.org/doc/first_steps/upgrade/index.en.html#index2h1 gives some explanations, but doesn’t provide step-by-step instructions.

I think it should look like:

1. Manually install a new Tails (link to “manual install from Linux)
2. Boot on the new manually installed Tails
3. Start Tails Installer and use the ”clone and upgrade" option to upgrade the old one, preserving persistence

Related issues

• Related to #10235 (closed)
• Is duplicate of #8874 (closed)