Multi-level Video Understanding - add swagger plugin

Author: wiwaszko-intel

microservices/multilevel-video-understanding/docs/user-guide/api-reference.md

@@ -2,6 +2,10 @@
 
 **Version: 1.0.0**
 
+<!--hide_directive```{eval-rst}
+.. swagger-plugin:: ./_assets/openapi.yaml
+```hide_directive-->
+
 When running the service, you can access the Swagger UI documentation at:
 
 ```bash