chatbot-rag-app: add instructions for k8s deployment

[codefromthecrypt]

Right now, there are instructions for chatbot-rag-app docker and python, but not k8s. @bshetti was interested in this.

k8s is more complicated as it requires a build step implicit when using docker-compose
(e.g. docker compose --build means you don't have to add a separate build image step)
Also, it needs to take note of all ENV variables set otherwise via .env in existing instructions

It may be possible that some layered tool over k8s can simplify this to the same easy experience as docker compose.

Meanwhile, folks should use normal k8s port forwarding as the example itself can access ES via localhost if the port is forwarded and doesn't need to be deployed inside k8s.

https://github.com/elastic/elasticsearch-labs/tree/main/example-apps/chatbot-rag-app