agent/exporters/kafka: update and enhance Apache Kafka page with visuals and other comments

[odeke-em]

It would be great if an Apache Kafka expert could take a look at https://opencensus.io/agent/exporters/kafka/ enhance that page with comments, updates and visuals to make it intuitive and useful for anyone who would like to send their traces to Apache Kafka.

/cc @yancl who created the trace exporter for Apache Kafka.