Rewrote the Kafka reference guide and getting started guide.

This should also include the Avro (with Apicurio) guides.

Apache Kafka Reference Guide: http://quarkus.io/guides/kafka

 Getting Started to SmallRye Reactive Messaging with Apache Kafka: http://quarkus.io/guides/kafka-reactive-getting-started

Using Apache Kafka With Schema Registry and Avro: http://quarkus.io/guides/kafka-schema-registry-avro