Include `sphinx_copybutton` extension for docs

[jsheunis]

sphinx_copybutton to be included in requirements-devel.txt and in the list of extensions in docs/source/conf.py

[adswa]

I indeed like this extension, and its very useful if docs have code-blocks with examples or instructions. As we had a discussion on bloating requirements and optional features before (#45), I wouldn't however be comfortable endorsing this alone

[jsheunis]

I endorse it too ;)