Better integrate new upgrade scenarios with upgrade documentation
Type of work:
For the release of the installation assistant, I did a quick fix but we could:
- Deduplicate doc/first_steps/upgrade/release_notes.inline.
- Merge /doc/first_steps/upgrade and /upgrade to prevent clicks and possible confusion.
- Rethink where Tails Upgrader should point to.
- Maybe mention release notes in IA.
Also, now that we are distributing USB images we should adapt /upgrade:
- It could be done equally from Windows and macOS than "from Linux" (as currently advertized)