docs: add common openedx settings to settings reference

Author: wgu-taylor-payne

docs/references/settings.rst

@@ -6,6 +6,12 @@ This is the list of (non-toggle) Django settings defined in the ``common.py`` mo
 .. note::
     Toggle settings, which enable or disable a specific feature, are documented in the :ref:`feature toggles <featuretoggles>` section.
 
+Open edX settings
+-----------------
+
+.. settings::
+    :folder_path: openedx/envs/common.py
+
 LMS settings
 ------------