Add high-level documentation for SinkBinding

[evankanderson]

Expected Behavior

I should understand what a SinkBinding is for and how to use it.

Actual Behavior

There's a sample that shows how to use it, but the way that it works and how to use it with my own container requires a little thought.

Steps to Reproduce the Problem

1. Read https://knative.dev/docs/eventing/ . There's nothing there that describes how to send events yourself.
2. Read https://knative.dev/docs/eventing/samples/sinkbinding/

   The SinkBinding can be used to author new event sources using any of the familiar compute abstractions that Kubernetes makes available (e.g. Deployment, Job, DaemonSet, StatefulSet), or Knative abstractions (e.g. Service, Configuration).
   That's cool -- but I don't really know how to use it.

3. Halfway down the page there's a single line that references $K_SINK and $K_CE_OVERRIDES. I wonder what those are and how they work.

As a start, we could pull in parts of the SinkBinding design doc as a top-level doc somewhere after Broker and Trigger. I'll also mention it in a rework of the top-level eventing docs page.

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