docs: Unpin docutils version (#1230)

Fixes bug during docs build at line 148, in MystParser
    validator: frontend.validate_smartquotes_locales,
AttributeError: module 'docutils.frontend' has no attribute 'validate_smartquotes_locales'

docs/requirements.txt

@@ -2,7 +2,7 @@ sphinx
 sphinxcontrib-napoleon
 sphinx-argparse
 sphinx_rtd_theme
-docutils==0.12
+docutils
 myst-parser
 configargparse
 appdirs