Pin docutils <0.17 to fix sphinx build with rtd theme

[zklaus]

Bullet lists on readthedoc are not formatted correctly due to an incompatibility between the readthedocs sphinx theme and newer versions of docutils. For details, see https://stackoverflow.com/a/68008428.

• Closes Bullet lists on readthedocs are not formatted correctly #2313
• Link to documentation: https://esmvaltool--2312.org.readthedocs.build/en/2312/

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[zklaus]

This is the same as ESMValGroup/ESMValCore#1320 for the tool.

[valeriupredoi]

and as with the Core one -> cheers @zklaus 😁 🍺