Include Reference Link to CAMARA API Design Guidelines in Notification Cloud Events Artifact

[PedroDiez]

Problem description

In current artifact:
https://github.com/camaraproject/Commonalities/blob/main/artifacts/notification-as-cloud-event.yaml#L37

It could be added the link to the documentation:
https://github.com/camaraproject/Commonalities/blob/main/documentation/API-design-guidelines.md#122-event-notification

Expected action

Include reference (highlighted below)

This endpoint describes the event notification received on subscription listener side when the event occurred. A detailed description of the event notification is provided in the CAMARA API design guideline document.

Additional context
Issue proposed for MetaRelease Spring 25 - #273

[rartych]

If we agree on API Design Guidelines cleanup: #306 then then link will change.

[rartych]

Decided to be resolved together with #306