Add Apache Kafka to Messaging section's description

Closes gh-30389

Author: wilkinsona

spring-boot-project/spring-boot-docs/src/docs/asciidoc/index.adoc

@@ -19,7 +19,7 @@ The reference documentation consists of the following sections:
 <<web#web,Web>> :: Servlet Web, Reactive Web, Embedded Container Support, Graceful Shutdown, and more.
 <<data#data,Data>> :: SQL and NOSQL data access.
 <<io#io,IO>> :: Caching, Quartz Scheduler, REST clients, Sending email, Spring Web Services, and more.
-<<messaging#messaging,Messaging>> :: JMS, AMQP, RSocket, WebSocket, and Spring Integration.
+<<messaging#messaging,Messaging>> :: JMS, AMQP, Apache Kafka, RSocket, WebSocket, and Spring Integration.
 <<container-images#container-images,Container Images>> :: Efficient container images and Building container images with Dockerfiles and Cloud Native Buildpacks.
 <<actuator#actuator,Production-ready Features>> :: Monitoring, Metrics, Auditing, and more.
 <<deployment#deployment,Deploying Spring Boot Applications>> :: Deploying to the Cloud, and Installing as a Unix application.