Kafka functions: user guide

[aliok]

Document how to create, configure, and deploy a Kafka function. Should cover:

• Creating a function with func create -l go -t kafka
• Configuring Kafka connection (env vars: KAFKA_BROKERS, KAFKA_TOPICS, KAFKA_CONSUMER_GROUP)
• Setting env vars via func.yaml (plain values, Secrets, ConfigMaps)
• Handler signature and return value semantics (nil = commit, error = skip)
• Optional lifecycle hooks (Start, Stop, Ready, Alive)
• Health check endpoints
• Deployment with min-scale: 1 requirement
• Scaling considerations (no HTTP traffic, consumer must stay running)