Doc Update: Clickhouse kafka connect performance tuning

[amolsr]

What's Changed

• Rewrote docs/integrations/data-ingestion/kafka/kafka-clickhouse-connect-sink.md with detailed coverage of:
• When performance tuning is actually needed (and when it's not)
• Detailed explanation of data flow through the connector
• Batch size configuration with recommended settings and trade-offs
• Complete section on asynchronous inserts including use cases, configuration, and important caveats about exactly-once semantics
• Connector parallelism and scaling strategies
• ClickHouse table engine considerations and settings
• Connection pooling and timeout configuration
• Monitoring key metrics and troubleshooting common issues
• Best practices summary
• Production-ready high-throughput configuration example

Closes ClickHouse/clickhouse-kafka-connect#595

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