Add a note that Apache Kafka is required to use EKB (#6404)

Author: creydr

docs/versioned/install/yaml-install/eventing/install-eventing-with-yaml.md

@@ -66,6 +66,9 @@ Follow the procedure for the Channel of your choice:
 
 === "Apache Kafka Channel"
 
+    !!! note
+        Access to an Apache Kafka cluster is required to use the Kafka Channel. You can do this for example by following the instructions on the [Strimzi Quickstart page](https://strimzi.io/quickstarts/).
+
     The following commands install the KafkaChannel and run event routing in a system
     namespace. The `knative-eventing` namespace is used by default.
 
@@ -120,6 +123,9 @@ Follow the procedure for the Broker of your choice:
 <!-- This indentation is important for things to render properly. -->
 === "Apache Kafka Broker"
 
+    !!! note
+        Access to an Apache Kafka cluster is required to use the Kafka Broker. You can do this for example by following the instructions on the [Strimzi Quickstart page](https://strimzi.io/quickstarts/).
+
     The following commands install the Apache Kafka Broker and run event routing in a system
     namespace. The `knative-eventing` namespace is used by default.