clean empty directories
Trac #16157: git clean empty doc dirs

Git ignores empty directories, but Sphinx does not. This can easily
break documentation build, and in particular #16050 does. And you can't
even get out of it with a "make distclean".

URL: http://trac.sagemath.org/16157
Reported by: vbraun
Ticket author(s): Volker Braun
Reviewer(s): Ralf Stephan