ENH: Add documentation generation for SlicerWizard

Add CMake machinery to generate (using sphinx) HTML documentation from
the Python docstrings of SlicerWizard. I've been using this internally
(calling sphinx-build by hand) for a while now, but am committing it now
that it has been tied into CMake. Parts of this are generated from
sphinx-apidoc, but there are also already some enhancements added, e.g.
intersphinx setup, and the overriding of autodoc.ClassDocumenter to
remove the redundant module name from class module classes.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22987 3bd1e089-480b-0410-8dfb-8563597acbee