Broker documentation needs cleanup

[abrennan89]

Expected Behavior

There's a clear place to find documentation on configuring brokers for the cluster or for a namespace.

Actual Behavior

• Broker documentation is scattered and hard to find throughout multiple places in docs.
• Broker documentation is divided by Operator config vs ConfigMap and it's not clear what the differences are or which a user would use in different setups.
• There is not enough documentation for non-default broker types, e.g. Kafka, other 3rd party brokers.

Steps to Reproduce the Problem

Conceptual broker info

• https://knative.dev/docs/eventing/broker/
• https://knative.dev/docs/getting-started/first-broker/

Developer broker docs

• Create default broker https://knative.dev/docs/eventing/broker/create-mtbroker/
• https://knative.dev/docs/eventing/broker/rabbitmq-broker/
• https://knative.dev/docs/eventing/broker/kafka-broker/
• https://knative.dev/docs/eventing/flows/sequence/sequence-with-broker-trigger/
• https://knative.dev/docs/eventing/observability/metrics/eventing-metrics/#broker-filter
• https://knative.dev/docs/eventing/event-delivery/#configuring-broker-event-delivery
• https://knative.dev/docs/eventing/sugar/#automatic-broker-creation
• https://knative.dev/docs/eventing/flows/sequence/#using-sequence-with-brokertrigger-model
• https://knative.dev/docs/eventing/sources/kafka-source/#connecting-to-a-tls-enabled-kafka-broker
• https://knative.dev/docs/eventing/accessing-traces/
• https://knative.dev/docs/eventing/event-registry/
• https://knative.dev/docs/getting-started/first-trigger/
• https://knative.dev/docs/eventing/sinks/kafka-sink/#installation
• https://knative.dev/docs/eventing/configuration/sugar-configuration/

Operator broker docs

• https://knative.dev/docs/install/operator/configuring-eventing-cr/#configuring-the-default-broker-class
• https://knative.dev/docs/install/operator/configuring-eventing-cr/#setting-the-default-channel-for-the-broker

Broker config (non-Operator)

• https://knative.dev/docs/eventing/configuration/broker-configuration/
• https://knative.dev/docs/eventing/broker/example-mtbroker/
• https://knative.dev/docs/eventing/sugar/#automatic-broker-creation
• https://knative.dev/docs/install/yaml-install/eventing/install-eventing-with-yaml/#optional-install-a-broker-layer
• https://knative.dev/docs/install/uninstall/#uninstalling-an-optional-broker-eventing-layer

Dev / other repo docs

• Broker spec https://github.com/knative/specs/blob/main/specs/eventing/overview.md#broker
• https://github.com/knative/specs/blob/main/specs/eventing/control-plane.md#broker-lifecycle

Fix

• [ ] Consolidate broker docs
• [ ] Create full, end to end user stories for admins and developers for each broker setup (more like tutorials)
• [ ] Create snippets as required for conceptual content reuse
• [ ] TBA