ENH: Add sphinx project files

Add necessary sphinx files to permit generating the Slicer Python
documentation with sphinx. Note that there are some hacks employed here,
as some of this code really doesn't like being imported outside of a
running Slicer instance.
ENH: Add build rules for sphinx documentation

Move files for sphinx documentation to Utilities/Sphinx, for consistency
with Utilities/Doxygen. Add build rules to invoke sphinx-build. Remove
unused reST file. Tweak module loading so that building the sphinx doc
does not depend on VTK's Python bindings (as (a) we don't have a way to
express this dependency, (b) setting up the paths to be able to import
the modules would be awkward, and (c) we shouldn't be trying to document
VTK anyway).
STYLE: Fix sphinx warnings; improve doc

Rewrite some bits of the Base/Python documentation that are triggering
sphinx warnings to use good sphinx markup. Add intersphinx so we can
cross-reference the base Python documentation. Fix documentation title.
Remove slicerqt-with-tcl documentation section, as sphinx won't process
it due to it being an invalid identifier.
STYLE: Fix redundancy in Python doc

Remove redundant (and confusing) specification of "slicer" package name
before inserting the autodoc, as it makes it look like there is a
slicer.slicer module.
STYLE: Improve slicerqt (Python) doc