Updated Serving API docs for 0.13.0 (#2271)

* Updated docs for eventing 0.13.0

* Adding updated eventing.md file for v0.13.0

* Add Serving API docs 0.13.0

* Adding Serving API docs for 0.13.0

* Update docs/reference/eventing/eventing-contrib.md

Co-Authored-By: Matt Moore <[email protected]>

Co-authored-by: Matt Moore <[email protected]>

Author: knative-prow-robot