Add spring-kafka dependency to spring-boot-docs for Javadoc links

See gh-6961

spring-boot-docs/pom.xml

@@ -560,6 +560,11 @@
 			<artifactId>spring-integration-jmx</artifactId>
 			<optional>true</optional>
 		</dependency>
+		<dependency>
+			<groupId>org.springframework.kafka</groupId>
+			<artifactId>spring-kafka</artifactId>
+			<optional>true</optional>
+		</dependency>
 		<dependency>
 			<groupId>org.springframework.security</groupId>
 			<artifactId>spring-security-config</artifactId>