doc(alerting): provide reference for webhook bodies

Created by: bobheadxi

Documents a subset of Alertmanager's webhook events that site admins can leverage for Sourcegraph alerts, as requested by a customer via @dadlerj

We exclude many fields because there are certain assumptions we can make about the grouping of alerts (for example, a guaranteed set of commonLabels, and that a group will only ever have one alert). I've moved the definition of these common labels to a separate variable to enable clearer documentation.