Document Tilt setup with another Kibana version

During development, it is sometimes necessary to test with the latest (or local) version of Kibana, for example in the following cases:
- dependency on a Kibana feature that isn't released yet, but might already be included in a snapshot version
- dependency on a Kibana feature that isn't available in a Docker image (like a local copy)

It is possible to modify the Tilt configuration by adding the following to `testing/infra/k8s/overlays/local/kustomization.yaml`:

```

- patch: |-
apiVersion: kibana.k8s.elastic.co/v1
kind: Kibana
metadata:
name: kibana
spec:
image: docker.elastic.co/kibana/kibana:8.8.0-SNAPSHOT
```

Where `docker.elastic.co/kibana/kibana:8.8.0-SNAPSHOT` should be replaced with the docker image name and version, or could even be a local-only docker image.

I think here we should at least document this possibility, and maybe provide a commented-out version directly into `testing/infra/k8s/overlays/local/kustomization.yaml` to make contributions easier.