Better integrate new upgrade scenarios with upgrade documentation
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)
Feature Branch: doc/10885-better-upgrade-doc