The source code for the CFD workbench is at https://github.com/qingfengxia/Cfd
and the source code for the CfdOF workbench is at https://github.com/jaheyns/CfdOF
. As far as I can tell, the associated documentation is in the README files on those GitHub pages–there doesn't appear to be anything additional in the general FreeCAD help wiki.
This post https://forum.freecadweb.org/viewtopic.php?f=10&t=2612
is a good place to start if you're interested in contributing to FreeCAD.
Both workbenches can be added to your FreeCAD through the Addon Manager, which is under the "Tools" menu. To work, they do require that you also have OpenFOAM on your computer, which you will need to install separately.