Add copybuttons to the notebooks

[nvaytet]

We should use this neat little sphinx extension to add buttons to copy and paste the code in cell in the notebooks in our documentation pages: https://github.com/executablebooks/sphinx-copybutton

Note that when i made a quick attempt, I was getting enormous buttons, just like the ones reported in this issue: executablebooks/sphinx-copybutton#69
It seems that it was very recently fixed with an update to the SVG, but until it makes it into a release, a temporary solution was to create a _static/copybutton.css file and specify a fixed size (16px) in there.

[nvaytet]

See also executablebooks/sphinx-copybutton#101