tarball man pages are too deep in the file hierarchy
gromacs.7.man installs from the tarball to something like $install/share/man/man7/man/gromacs.7 iso install/share/man/man7/gromacs.7
Improved man-page machinery
Simplified management of MAN_PAGE_DIR, and reordered code in
docs/CMakeLists.txt to keep similar content together.
Added some comments.
Defaulted to not building help when cross compiling.