[WIP] Clean up and group kafka docs

[abrennan89]

Proposed Changes

• Move Kafka to own section under eventing
• Group existing kafka component docs, identify gaps as comments
• Edit, review, rewrite, restructure docs

Direct preview links:

• https://deploy-preview-5210--knative.netlify.app/docs/eventing/kafka/kafka-install/
• https://deploy-preview-5210--knative.netlify.app/docs/eventing/kafka/kafka-source/
• https://deploy-preview-5210--knative.netlify.app/docs/eventing/brokers/broker-types/kafka-broker/
• https://deploy-preview-5210--knative.netlify.app/docs/eventing/kafka/kafka-sink/
• https://deploy-preview-5210--knative.netlify.app/docs/eventing/kafka/kafka-channel-configuration/
• https://deploy-preview-5210--knative.netlify.app/docs/eventing/sinks/
• https://deploy-preview-5210--knative.netlify.app/docs/eventing/triggers/

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[knative-prow-robot]

@abrennan89: PR needs rebase.

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