Documentation - Configure RTD to build all offline formats

- https://docs.readthedocs.io/en/stable/config-file/v2.html#formats

Fixes #12484

.readthedocs.yml

@@ -9,5 +9,6 @@ python:
       path: .
       extra_requirements:
         - docs
+formats: all
 sphinx:
   fail_on_warning: True