So, you want to build the manual? This is now easy.

1. Configure the main GROMACS repo with GMX_BUILD_MANUAL=on
2. cmake will run some detection about whether this is possible
3. Later, use "make manual," which will build GROMACS in the
   usual way, and use the generated binaries to build the manual.

An installation of pdflatex and ImageMagick is assumed by the
UseLATEX.cmake package (https://gitlab.kitware.com/kmorel/UseLATEX)
that is used by manual/CMakeLists.txt.
