pin Sphinx version for RTD

[mazweg]

There is a problem with Sphinx version 1.7.1+ in RTD (#3769). The solution is to pin the Sphinx version. Right now this is not possible when using a conda environment in RTD (#3829). Therefore we will use the doc_requirements.txt file to pin the version for now.

I also created a fallback branch to go back to the conda environment as soon as either Sphinx is fixed or they added the feature to pin versions in the RTD conda environment.