docs: Migrated the stages component from the old docs to the new docs

Author: erinlefeyijimi

docs/index.rst

@@ -166,6 +166,7 @@ There are several ways to support Mautic other than contributing with code.
    rest_api/point_groups
    rest_api/reports
    rest_api/text_messages
+   rest_api/stages
 
 .. toctree::
    :maxdepth: 2