Update "See also:" section links in what_is_jupyter.rst

[yesthesoup]

I am learning Jupyter and wanted to click some links, but they are 404-ing. I have updated the two links that still exist from Jupyter's github + removed one that hasn't been updated in 6+ years.

https://github.com/OpenTechSchool/python-data-intro/tree/gh-pages/core

I can add it back if we still think it's valuable though, probably the rest of the links / the official doc covers most of it though.