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

Original created by @sajolida on 10885 (Redmine)

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