docs: Add link to new ODS repo OpenDataServices/os4d#49

Author: jpmckinney

docs/standard/schema.rst

@@ -16,3 +16,7 @@ Our extensions to JSON Schema
 -----------------------------
 
 We add to JSON Schema the properties ``codelist``, ``openCodelist``, ``deprecated``, ``omitWhenMerged`` and ``wholeListMerge``, documented in `Standards Lab <https://os4d.opendataservices.coop/development/schema/#extended-json-schema>`__ and defined in a `metaschema patch <https://github.com/open-contracting/standard/tree/HEAD/schema/metaschema>`__.
+
+.. seealso::
+
+   `Open Data Services JSON Schema Extension <https://json-schema-extension.readthedocs.io/latest/>`__