The doxygen documentation is generated using Engine/doc/doxygen/createDocumentation.sh. This script seems to get stuck (I stopped it after 15 minutes) at the file Engine/include/core/morphing/vbp_routines.h, which is generated by a Python script during the build, and has a size of 180k+ lines and 13+ MB. If that file is not present, the generation of the doxygen documentation takes only a couple of seconds.
It would be great if generating the documentation always worked, irrespective of whether that file is present or not. It would be ok to ignore that file in the documentation.