docs: warn that ksqlDB stream and Kafka topic names must match (case-sensitive)

[mintlify]

Strengthens the warning in the Kafka streams mode → Topic name matching section to make it very obvious that:

• The ksqlDB stream/table name and the backing Kafka topic name must be identical
• The match is case-sensitive
• Users should explicitly set KAFKA_TOPIC= when creating the stream to ensure the names match

Also adds a CREATE STREAM example showing how to set the topic name explicitly.

Requested in Slack by Julio Avila.

[mintlify]

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