fix requirements.txt for docs
I ran into a problem installing requirements.txt in the docs
folder:
ERROR: Cannot install -r docs/requirements.txt (line 38), -r docs/requirements.txt (line 44) and docutils==0.18 because these package versions have conflicting dependencies.
The conflict is caused by:
The user requested docutils==0.18
recommonmark 0.7.1 depends on docutils>=0.11
sphinx 4.2.0 depends on docutils<0.18 and >=0.14
#970 (closed) is related to prevent this in the future.