Build: distribute doc directory
Makefile in doc subdirectory should be integrated into the main build
system but I do not have time to do it now. For now it is enough
to distribute everything.
https://fedorahosted.org/freeipa/ticket/6418
Reviewed-By: Martin Basti <mbasti@redhat.com>
Reviewed-By: Christian Heimes <cheimes@redhat.com>