docs: Unpin docutils

[joverlee521]

Prompted by #1126

Unpinning docutils to be able to pull in the latest sphinx-argparse to fix doc builds.

Release checklist

internal updates only

[joverlee521]

From 567247e that pinned docutils ~2 years ago:

docutils 0.18 does not seem to install well with Python 3.10. Pinning to
earlier versions seems to fix the issue.

This is no longer an issue in the docs-ci, which is using the current default Python 3.12.
Does this affect anyone building docs locally?

[victorlin]

I just created the docs environment locally with this PR change and a Python 3.10 pin. docutils resolved to version 0.20.1 which works fine. If anyone experiences issues, they should be able to fix by recreating the environment from conda.yaml.