MINOR: Update introduction page in Kafka documentation 
MINOR: Update introduction page in Kafka documentation 

Although APIs section in Kafka documentation lists 5 core APIs (https://kafka.apache.org/documentation/#api), introduction page in Kafka documentation lists 4 of them. I've added the missing list element to fix this incoherence.

Reviewers: Manikumar Reddy <5c5b03b4d6e1a3a45d39ad15b70eab568fe0a48b@gmail.com>
guozhangwang/kafka,Chasego/kafka,apache/kafka,sslavic/kafka,guozhangwang/kafka,TiVo/kafka,lindong28/kafka,apache/kafka,lindong28/kafka,guozhangwang/kafka,Chasego/kafka,lindong28/kafka,lindong28/kafka,apache/kafka,sslavic/kafka,Chasego/kafka,TiVo/kafka,sslavic/kafka,TiVo/kafka,guozhangwang/kafka,sslavic/kafka,apache/kafka,TiVo/kafka,Chasego/kafka