chore: limit docutils to <0.21.0

sphinx-rtd-theme currently does not support docutils 0.21.x.

This restriction can be lifted once the upstream issue has been resolved:
readthedocs/sphinx_rtd_theme#1557

.github/renovate.json

@@ -12,6 +12,10 @@
     {
       "matchBaseBranches": [ "/^v[0-9]+/" ],
       "groupName": "all dependencies"
+    },
+    {
+      "matchPackageNames": ["docutils"],
+      "allowedVersions": "<0.21.0"
     }
   ]
 }