There do appear to be some docs embedded in the source.

https://github.com/GaijinEntertainment/DagorEngine/tree/main...

I haven't built them/don't know how complete they are. At a glance I would guess not very.

There is a bunch of *.rst files which are readable. On Ubuntu ReText opens them. Build/install probably just moves them around and/or may be converts to html . There is not much in there.