/doc/ does not display all Seldon protocol endpoints

[FarrandTom]

Describe the bug

The /doc/ endpoint provides a swagger schema for the endpoints implemented by the Seldon protocol, however it is incomplete. Currently, it only displays /prediction and the /feedback endpoints. This is therefore missing the /doc/ endpoint itself, the /metadata and the /health/status endpoint.

To reproduce

1. Deploy any Seldon Core model.
2. Navigate to the /doc/ endpoint as per this format: http://<ingress_url>/seldon/<namespace>/<model-name>/api/v1.0/doc/
3. There will only be the /metadata and /feedback endpoint displayed there.

Expected behaviour

The other Seldon protocol endpoints which are defined will also be displayed.