Docs: Remove ipython intersphinx mapping

The old link, https://ipython.org/ipython-doc/stable, no longer works. There is a new one: https://ipython.readthedocs.io/en/stable/, but we don't actually need it, so it can be removed.

However, removing it pointed out a bad reference - we don't have a link called tutorial, but ipython does, so our docs were incorrectly linking to that! The link is fixed now: tutorial-guide.