A runbook is a set of instructions for a human what to do. More specifically what to do when a certain monitoring alert triggers.
Prometheus alerting rules link to this page in a service-specific anchor based on the service name itself; from the service entry you can give more context, next actions, and link to other resources such as more runbooks, service dashboards, etc.
There is a Category:Runbooks for pages with runbooks.
This is the entry linked from generic alerts for service-name:port, for example ProbeDown will link here when network probes for service-name fail. See also bug T312947.
This is the service powered by chartmuseum, see ChartMuseum