Document Trigger Delivery Spec aspects

[matzew]

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

[snneji]

/priority medium

[github-actions]

This issue is stale because it has been open for 90 days with no
activity. It will automatically close after 30 more days of
inactivity. Reopen the issue with /reopen. Mark the issue as
fresh by adding the comment /remove-lifecycle stale.