[DOCS-39462]-Use Schema Registry in the README producer example

[Jamie Gillenwater (jgillenwater)]

Summary

• Change the README's first producer example from producing a raw UTF-8 string to producing through a Schema Registry JSONSerializer.
• Add a StringSerializer for the key and a SerializationContext for the value, matching the pattern in examples/json_producer.py.
• Add a trailer noting the confluent-kafka[json,schemaregistry] extra, and linking to the Schema Registry Integration section and the fuller examples/json_producer.py.

Addresses DOCS-39462: the first example a developer copies sets the default. Producing raw bytes leads to data-quality issues, broken consumers, and ungovernable data, so the README's first-run example should use Schema Registry. This is one of five coordinated PRs across librdkafka, confluent-kafka-python, confluent-kafka-go, confluent-kafka-dotnet, and confluent-kafka-javascript.

Verification

• Example verified by an SME against a local broker and Schema Registry.
• Snippet extracted from the rendered README and checked with py_compile.

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