The following are a list of helpful links and current engines for modding Quake today. The links were invaluable to me as I developed battleMETAL and they will certainly help you when digging into the code for battleMETAL.
QuakeC tutorials on ModDB.
these are the most popular forks of the Quake 1 engine
(Most modding of Quake engines is for Quake 1 and using these forks)
Specific Files in Darkplaces source:
both of these files have documentation in them that mostly describes how to use the Darkplaces engine extensions.