Hosting documentation on readthedocs.

[alxmrs]

Updating sphinx docs and adding a readthedocs.yml file.

Fixes #137.

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[alxmrs]

@naschmitz if you make an account on readthedocs.org, I'll add you as a project maintainer.

[12rambau]

Let me knox when this is merged so I can start playing with the documentation set-up. BTW can you set-up a PR hook as well so we see pre-build when PR are created ?