Kafka functions: E2E testing guide

Document how to test Kafka functions end-to-end on a local cluster:

- Setting up a kind cluster with Knative Serving
- Installing Kafka (e.g. Strimzi)
- Creating topics, deploying the function, sending messages, verifying logs
- Manual build workaround while `knative.dev/func-go/kafka` is unpublished
- Troubleshooting common issues (scaling to zero, missing env vars, connection failures)

Start by locating the repository's existing documentation and E2E testing guidance, then review the Knative Serving, kind, and Kafka setup described in the issue. Document the local-cluster workflow, the manual build workaround for knative.dev/func-go/kafka, and the listed troubleshooting cases. Done means a reader can deploy a Kafka function locally, send messages, verify logs, and diagnose common failures.

Written by the indexing model from the issue text.