STYLE: Document SlicerWizard package

Add some (very basic) documentation of the SlicerWizard package. This is
generic documentation of the package itself, different from the already
existing documentation of individual classes and modules within the
package. In particular, add a short note that this is API documentation
and not documentation for users of the Extension Wizard (with a link to
that documentation).

Also, remove mention of "Extension Wizard" in summary documentation of
WizardHelpFormatter, to avoid implying it is only useful for that
purpose.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22999 3bd1e089-480b-0410-8dfb-8563597acbee