ENH: Refactor documentation targets

Replace generate_userdoc.cmake script with direct build commands,
separating creation of the doc and creation of the tarball thereof into
separate targets, for consistency with how the SlicerWizard doc build
rules are set up. Also modify the latter to explicitly remove the old
tarball first for consistency with (the historic and retained behavior
of) the former.

The old rules depended on the output index.html, which would cause them
to do nothing once the doc had been created (ever). The new rules, like
the rules for the SlicerWizard doc, have no dependencies and so the doc
is always regenerated when the target is traversed. As doxygen
unfortunately does not implement its own dependency tracking (sphinx
does), this is very inefficient, but more correct, which is more
important for this case.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23210 3bd1e089-480b-0410-8dfb-8563597acbee