Add attributes filter documentation for Triggers

[nachocano]

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

As of 0.8, we are deprecating the sourceAndType filter on Triggers, and adding a replacement called attributes.

How will this impact our users?

It won't, they will have more functionality available. In a next release, sourceAndType will be deleted, so they should be able to migrate to attributes

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

0.8

Context

Link to associated PRs or issues from other repos here.

1. Attributes filters in Triggers eventing#1643
2. Allow Trigger Filter to filter on any Cloud Event Attribute (except data) eventing#1610

Additional info

[nachocano]

/assign