embed plots in user guide and quick start guide

[amueller]

Right now the user guide doesn't have any plots except for the examples.
We should be able to render the plots generated by the doctests somehow.
Maybe
https://matplotlib.org/sampledoc/extensions.html
or
https://matthew-brett.github.io/nb2plots/nbplots.html

[amueller]

Hm we can use the matplotlib plot directive, but that seems to require either duplicating all the code or putting everything in one directive? It's a bit confusing.
Maybe using sphinx-gallery is easier?

[amueller]

done