Add documentation for Authorization support in Knative-Eventing

[creydr]

What is changing?

Knative Eventing will support Authorization via EventPolicies. The feature track is available here.

How will this impact our users?

Users can define EventPolicies to restrict who can send events to a (knative-eventing owned) Sink.

In what release will this take happen (to the best of your knowledge)?

1.16

Context

Project: https://github.com/orgs/knative/projects/79/views/1

Additional info

[creydr]

/assign