At some point I renamed doc into Documentation but there are projects
out there who test the presence of doc/ to build 'zio'.

We should fix the Makefile in all these projects but right now it is not
that easy and this is the easiest fix to make the CERN COHT building mechanism
working again.
Signed-off-by: federico.vaga@cern.ch <federico.vaga@cern.ch>