Host docs using Github Pages

[ranisalt]

I see that documentation is available at /docs. Coincidentally, that's exactly one of the options where Github can automatically serve pages:

Please do this, so that documentation is available at pdfminer.github.io/pdfminer.six

[pietermarsman]

I've started working on sphinx documentation on the documentation branch. It is published on readthedocs.

Keep in mind: work in progress, help is appreciated.