Add docutils as a dependency

[markusdregi]

docutils is explicitly needed to run the sphinx extension. Currently sphinx as well depend on docutils so it is not much of a problem if you are to use the extension, but we should add it as a dependency in setup.py (it is already listed in ci-requirements.txt).

An alternative, if we don't want to force people to install all dependencies of all extra functionalities of config suite unless they want them is to introduce extras in the project.