It is very important that systems are up-to-date prior to upgrading. Mention
that several times in the guide, and also indicate that on the screenshot.
* Improve messaging regarding N->N+2 upgrades.
* Move some details regarding Rawhide issues to the specific Rawhide wiki page.
This generic guide doesn't need to go into detail about Rawhide issues,
because that makes it harder to read for general users (not interested in
Rawhide). Power users can follow a link.
* Update gnome-software screenshot (more recent, with graphics).
* Clearly state that upgrading using pure `dnf` or `fedora-upgrade` is
unsupported.
* Other small adjustments and clarifications, link fixes. Make section headlines
look consistent across articles (don't capitalize every word).
