use nbstripout

[prjemian]

The notebooks like this one are already rendered, which makes it hard to review as there are huge image diffs there. I think we can use https://github.com/kynan/nbstripout and auto-render the notebooks during the sphinx docs building process using https://nbsphinx.readthedocs.io/en/0.9.3/.

Originally posted by @mrakitin in #300 (comment)