docs: Finalize selective replication launch surface

[rahulbsw]

Summary

• reposition the public launch surface around selective replication for Kafka and Redpanda
• add comparison, compatibility, quickstart, and example docs for the promoted use cases
• validate the promoted example configs in CI, including Redpanda and production-focused examples

Verification

• cargo test --quiet
• cargo run --quiet --bin streamforge-validate -- examples/configs/config.example.yaml
• cargo run --quiet --bin streamforge-validate -- examples/redpanda/selective-replication.yaml
• cargo run --quiet --bin streamforge-validate -- examples/production/pii-redaction.yaml
• cargo run --quiet --bin streamforge-validate -- examples/production/cdc-to-datalake.yaml