Add recommonmark to the list of required Sphinx extensions

[mwouts]

This PR fixes #344 .

With this small change, the local rendering of the docs is much better, both for the index, and for the feature page (compare the screenshots below to those on #344 to see the improvement)

[kevin-bates]

Thanks Marc!

[kevin-bates]

hi Marc - thanks for the contribution.

I've been looking at the build failures and have found they are related to a traitlets version increment from (at least) 4.3.3 to 5.0.4. I can reproduce the test failure in using the pip freeze output of the failing python 3.8 tests. In diffing the same output from the python 3.6 tests, there are several differences, but traitlets seemed the most suspicious to me and I was right. Reverting to traitlets 4.3.3 in py 3.8 allowed the test to pass.

I have not determined exactly why traitlets is affecting the swagger facility (there are several test failures, but the CI must be set up to stop on first failure). Given the simplicity of this change and the fact that it introduces nothing related to the core package and its tests, I'm going ahead with the merge.

We will probably need to cap traitlets in JKG for now, but that's a different PR (and perhaps even shorter than this one! 😄 ).

[mwouts]

Thank you Kevin. I'm sorry to see that this PR gives you much more work than expected! Also I see that the doc status is failing , maybe we need to change something on the RTD website as well?

To compile the docs locally I have used

cd docs
conda env create --file environment.yml
conda activate kernel_gateway_docs
make html