fix(docs): restore theme

Broken in 190bae5b70f035ce0b45770b2fcecfb6ddc19f2a

.readthedocs.yaml

@@ -5,6 +5,10 @@ build:
   tools:
     python: "3.11"
 
+python:
+  install:
+    - requirements: docs/requirements.txt
+
 # Build documentation in the "docs/" directory with Sphinx
 sphinx:
   configuration: docs/conf.py

docs/requirements.txt

@@ -1 +1 @@
-docutils<0.21
+sphinx-rtd-theme