docs: add RFC 0004 for Kafka to Iceberg export

[yatharthranjan]

Summary

• Add RFC 0004 in rfcs/platform/0004-streaming-radar-kafka-to-iceberg-s3.md for production-ready Kafka-to-Iceberg export architecture on RADAR-Kubernetes.
• Specify concrete implementation guidance including the community Iceberg Kafka Connect sink (org.apache.iceberg.connect.IcebergSinkConnector), published-first deployment artefact policy, and a detailed rollout sequence.
• Add architecture diagram and update RFC index in README.md.

Test plan

• Verify RFC file renders correctly in Markdown, including Mermaid diagram.
• Confirm RFC numbering/title/metadata and index entry are consistent.
• Validate links to Helm charts and connector references are present.
• Review content with maintainers for final operational SLO values and image pinning decisions.