STYLE: Move WizardHelpFormatter to its own module

As the class is referenced in the SlicerWizard.TemplateManager
documentation, and as ExtensionWizard is a class module which (as a
module) we want to hide, move WizardHelpFormatter to its own module so
that it is available to external users. Also, add documentation for the
same.
STYLE: Remove execute permissions from modules

Remove the +x bits from the SlicerWizard module .py files, as these are
not needed or wanted (and now that the shebangs are removed, the commit
hook doesn't like them).
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.