Improve the doc formating on RTD

[mwouts]

Hi,

Thanks for this nice project!

I see that the documentation uses Markdown files, but these files are not properly rendered on RTD.

For instance, this file: https://github.com/jupyter/kernel_gateway/blob/master/docs/source/features.md

is rendered on https://jupyter-kernel-gateway.readthedocs.io/en/latest/features.html as

In my experience this can be fixed by activating the recommonmark extension in docs\conf.py.

Would you be open to a PR going into that direction?

[kevin-bates]

Absolutely! Thank you Marc!

[mwouts]

Done! And that is possibly the shortest PR I ever made 😄
Thank you Kevin for you quick answer.