Moving CommonMark-py to readthedocs

[lu-zero]

Recently readthedocs kindly asked me if I'd mind to move recommonmark to their organization and I accepted, I'd move the development of CommonMark-py there as well, you'd be ok with that?

[ericholscher]

We're hoping to fully support Commonmark & Recommonmark on Read the Docs, and take some responsibility in maintenance.

We would of course give you full credit as the author, and commit to the repos. We're also happy to keep development here; we're just trying to provide a solid commonmark stack on our site, and increase development effort in the dependencies.

[lu-zero]

@ericholscher since there is no activity I'd move it.