Dead link to Broker spec at `Eventing` / `Brokers` / `About Brokers`

[antoineco]

Expected Behavior

At https://knative.dev/docs/eventing/broker/#alternative-broker-implementations, the link named broker specifications should be valid and point to the Broker specification at https://github.com/knative/specs/blob/f3dcd525/specs/eventing/control-plane.md#broker-lifecycle.

Actual Behavior

It points to https://github.com/knative/specs/blob/main/specs/eventing/broker.md, which was merged in knative/specs#25 with https://github.com/knative/specs/blob/f3dcd525/specs/eventing/control-plane.md.

Steps to Reproduce the Problem

1. Visit https://knative.dev/docs/eventing/broker/#alternative-broker-implementations
2. Click broker specifications
3. Be prepared for an unpleasant surprise

Additional Info

I would highly recommend GitHub links to include revisions to avoid this kind of behaviour.

[antoineco]

/assign