exclude multi-entity association endpoint from api (#527)

We're reworking this endpoint entirely, probably removing it from
SolrImplementation at some point.
So it doesn't make sense for it to show up in the swagger UI

backend/src/monarch_py/api/association.py

@@ -41,7 +41,7 @@ async def _get_associations(
     return response
 
 
-@router.get("/multi")
+@router.get("/multi", include_in_schema=False)
 async def _get_multi_entity_associations(
     entity: Union[List[str], None] = Query(default=None),
     counterpart_category: Union[List[str], None] = Query(default=None),