Document debusine.debian.net account synchronizations shortcomings and setup support contact point

Until the synchronization feature requested in #976 is completed, we probably should document this lack of synchronization in https://wiki.debian.org/DebusineDebianNet and have some official contact point to reach administrators of debusine.debian.net (maybe a dedicated salsa project like the salsa admins have their support project) so that we can manually fix the permissions when required.