Pin docutils <0.17 to fix bullet lists on readthedocs

[zklaus]

An incompatibility between the readthedocs sphinx theme and newer versions of docutils prevents proper rendering of bullet lists.
By pinning docutils <0.17 in the doc environment, we can get proper lists again.

For details see https://stackoverflow.com/a/68008428.

Description

Pins docutils <0.17.

Closes #1179

Link to documentation: https://esmvaltool--1320.org.readthedocs.build/projects/ESMValCore/en/1320/

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[codecov]

Codecov Report

Merging #1320 (d9a9ba5) into main (723cd8f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1320   +/-   ##
=======================================
  Coverage   86.69%   86.69%           
=======================================
  Files         188      188           
  Lines        9190     9190           
=======================================
  Hits         7967     7967           
  Misses       1223     1223           

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[valeriupredoi]

cheers @zklaus 🍺