Docs: Document Kafka Connect control topic purpose and retention

[wombatu-kun]

Summary

Users have reported confusion about the Kafka Connect sink's control topic growing without bound (#15844, #15844). The docs explain how to create the control topic but never describe what it is for, how its events are used, or that it should have a finite retention — so on brokers with a large or unlimited default retention.ms, the topic accumulates coordination events indefinitely. This documents the behavior and the recommended configuration.

What changed

Expanded the "Control topic" section of docs/docs/kafka-connect.md:

• Added a paragraph on the control topic's purpose and the per-commit event flow (StartCommit / DataWritten / DataComplete / CommitToTable / CommitComplete), noting that DataWritten carries data/delete file metadata rather than rows, and that the durable commit position lives in the table snapshot — so control-topic events are transient.
• Added --config retention.ms=3600000 to the topic-creation example.
• Added a "Control topic retention" subsection: why an auto-created topic grows under broker defaults, how to size retention.ms relative to the commit interval/timeout, how to set it on an existing topic, multi-connector sizing, and using cleanup.policy=delete rather than compaction.

Closes #15844