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.