Kafka functions: user guide

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)

Start by locating the user-guide structure and the Kafka function flow created by `func create -l go -t kafka`. Cover the listed Kafka environment variables, `func.yaml` values, handler and lifecycle semantics, health checks, deployment, and scaling guidance. Done means a complete guide that includes the `min-scale: 1` requirement and all requested examples.

Written by the indexing model from the issue text.