Document and update examples for v1beta1 for Broker,Trigger, and EventType

[vaikas]

What is changing? (Please include as many details as possible.)

Adding v1beta1 apis for Broker,Trigger, EventType

How will this impact our users?

Users will have a cleaner API.

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

V0.13

Context

Link to associated PRs or issues from other repos here.

1. Trigger: Add trigger v1beta eventing#2414
2. EventTypes: Eventtypes v1beta1 eventing#2466
3. Broker: Broker v1beta1 eventing#2449

Additional info

[abrennan89]

@vaikas is this issue closed by #2259 ? If not, what else is required here?