[gitlab-ci] only trigger readthedocs on branch/default

.gitlab-ci.yml

@@ -20,6 +20,9 @@ check-manifest:
   script: tox -e check-manifest
 
 trigger-other-pipelines:
+  only:
+    refs:
+      - branch/default
   except:
     variables:
       - $TRIGGERED_FROM_OTHER_PROJECT