Move everything into intro-HPC.

Makefile works like this:
make -> complains that you should specify SITE, OS and DOC.

make DOC=intro-HPC OS=mac SITE=gent -> builds intro-HPC document for ghent, mac version.
make DOC=intro-HPC -> builds all versions for all os'es and sites.
make DOC=intro-HPC SITE="gent leuven" OS=mac -> build mac versions for gent and leuven.

@boegel Do you approve of the above?