Generate documentation automatically

[mtazzari]

Generate documentation automatically, e.g. using a tool like sphinx, embedding the docstrings provided in the code.
Have a look at how we do it in galario: https://github.com/mtazzari/galario/tree/master/docs

It needs #3 to generate docs automatically after each push to master

[mtazzari]

Requires the repo to be public in order for doctr tool to pull the code and push to github pages.

[mtazzari]

Fixed by #23