Document Trigger Delivery Spec aspects

**Describe the change you'd like to see**

The documentation on "Event Delivery" does not have any example on its usage for triggers.
Example code:
https://github.com/knative/eventing/blob/d5f8b90a0050dac74ec534da3d84468496f38bd7/test/rekt/resources/trigger/trigger_test.go#L236-L274

**Additional context**
Add any other context or screenshots about the feature request here.

Looking here: https://knative.dev/development/developer/eventing/event-delivery/

I don't see a mention of Triggers, than jumping here https://knative.dev/development/eventing/broker/triggers/

or https://knative.dev/development/developer/eventing/event-delivery/

I am also not seeing anything

Compare the Event Delivery page at knative.dev/development/developer/eventing/event-delivery/ with broker/triggers/ and trigger_test.go lines 236-274. Document the missing Trigger delivery aspects and usage example, then verify the links and rendered documentation.

Written by the indexing model from the issue text.