Missing documentation about desktop-focused software repos

[jonasrosland]

As discussed in Mattermost regarding the default software repos, when going into "Software", there aren't a lot of applications available. When going into the "Play" category for instance, it's blank.

Sokel mentioned enabling epel-release, and Skip mentioned adding Flathub as well.

If not shipped with these enabled by default, it should be documented how to easily enable these.