PUBDEV-8362: updated sphinx versions

[NidhiMehta]

1. Will need to create a new docs docker image (not sure if will be created automatically) ( https://github.com/h2oai/h2o-3/blob/master/docker/Dockerfile.dev)
2. Need to make sure pip is using python version 3.5(or later ) (not sure from here - https://github.com/h2oai/h2o-3/blob/master/docker/scripts/install_python_version)
3. Need a way to test locally the jenkins way with a make docs_in_docker command. make sure the syntax substitutions etc are working correct
   cc @hannah-tillman

[michalkurka]

@valenad1 idea: instead of investigating what is going wrong with the images - maybe it would be easier to have a specific python environment just for building the documentation (and update the build scripts)