This groups the files and templates for each host together and therefore
makes it easier to see all the (config) files for a host.
Also clean up incorrect, unused docker_compose config for mumble and
clean up unused engelsystem configs.
For now introduce node-exporter/hosts alert rules, which got taken from
https://samber.github.io/awesome-prometheus-alerts/rules
However with the labels removed from the description, since they don't
render correctly (at least in Telegram) and don't seem to provide much
value, as we render the labels in the notification anyway.
Also only have Telegram as the notification channel for now, as it was
the easiest to set up.
