[DOC] Documenting Kafka Sink plugin

[alexstrive]

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Inspecting the code, I have found Kafka sink plugin
https://github.com/opensearch-project/data-prepper/blob/main/data-prepper-plugins/kafka-plugins/src/main/java/org/opensearch/dataprepper/plugins/kafka/sink/KafkaSink.java

According to documentation, there're no such sink: https://opensearch.org/docs/latest/data-prepper/pipelines/configuration/sinks/sinks/

I'm not sure whether it's feature-ready, or for internal use only (e.g. for Kafka buffer). But looks like a proper plugin. When I create pipeline with such sink – DataPrepper doesn't crash because there are no such plugin.

I think it must be documented. Or at least discussed whether we need to document it. I didn't find any information about it in this issues section.

Version: List the OpenSearch version to which this issue applies, e.g. 2.14, 2.12--2.14, or all.

DataPrepper documentation - all.

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.

DataPrepper's KafkaSink class with plugin annotation - https://github.com/opensearch-project/data-prepper/blob/main/data-prepper-plugins/kafka-plugins/src/main/java/org/opensearch/dataprepper/plugins/kafka/sink/KafkaSink.java
DataPrepper documentation - https://opensearch.org/docs/latest/data-prepper/pipelines/configuration/sinks/sinks/