update system upgrade docs
    
    * 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).