try ubuntu-20.04, which matches vcs_environment?

.readthedocs.yml

@@ -11,14 +11,19 @@ python:
    - requirements: requirements/user-docs.txt
 
 build:
-  os: ubuntu-22.04
+  os: ubuntu-20.04
   tools:
     python: "3.11"
-  jobs:
-    pre_system_dependencies:
-      - |
-        if [ "$READTHEDOCS_VERSION_TYPE" = "external" ] && git diff --quiet origin/main -- docs/ .readthedocs.yml requirements/docs.txt;
+  commands:
+    - |
+        if [ "$READTHEDOCS_PROJECT" = "warehouse" ]
         then
-          echo "No changes to docs/ - exiting the build.";
-          exit 183;
+          make -C docs/ html;
+          mv docs/_build/html _readthedocs/html;
+        fi
+    - |
+        if [ "$READTHEDOCS_PROJECT" = "docspypiorg" ]
+        then
+          mkdocs build -f user-docs/mkdocs.yml;
+          mv user-docs/site _readthedocs/html;
         fi