It is not clear why Eventing optional components are optional

Users have been reporting multiple times that their Broker or Channel setup doesn't work because the documentation reports that Broker and Channel layers are optional like in https://github.com/knative/eventing/issues/5944.

Those optional components are required to follow the next steps, so I'm proposing to remove the Optional word in the heading and instead add a sentence explaining why a Broker or Channel layer might be optional and what you can't do without that particular layer.

## Expected Behavior

- There is no confusion

## Steps to Reproduce the Problem

1. Install required components only
1. Follow the create a Broker page: https://knative.dev/docs/eventing/broker/create-mtbroker/

## Additional Info
- https://knative.dev/docs/install/eventing/install-eventing-with-yaml/
- https://knative.dev/docs/install/eventing/install-eventing-with-yaml/#optional-install-a-default-channel-messaging-layer
- https://knative.dev/docs/install/eventing/install-eventing-with-yaml/#optional-install-a-broker-layer

**Install information:**
- Platform (GKE, IKS, AKS, etc.): All
- Knative Version: All

/kind eventing

Start with the installation documentation sections linked in the issue, especially the anchors for the optional default channel messaging layer and broker layer. Review the create-a-Broker page to confirm which next steps depend on each layer. Done means removing the misleading “Optional” wording and adding clear explanations of when each layer is optional and what is unavailable without it.

Written by the indexing model from the issue text.