Synchronize the single-application mode with RHEL
When updating the single-application documentation of RHEL, we also found out that the GNOME documentation on the same task is outdated and no longer works.
This MR updates the GNOME single-application documentation to use the configuration files that have been distributed with RHEL 8.
In RHEL 9, they're now replaced with a binary program that provides the same functionality, but unfortunately it's not possible to copy a binary in documentation, so I'm using the older configuration, which still works.
@pmkovar, could you please give me some pointers on how to get this MR reviewed and merged?