docs: simplify script to use kubectl exec inside the pod

Rewrote the script to run commands inside the runtime-api pod via kubectl exec:

- No longer requires external Runtime API URL
- No need for curl or python3 installed locally
- Only prerequisite is kubectl access to the cluster
- Script finds the runtime-api pod automatically
- Runs Python inside the pod (which already has Python installed)
- Uses localhost:5000 to connect to the API from within the pod

This is much simpler for administrators since it works regardless of
whether the Runtime API has external ingress configured.

Co-authored-by: openhands <openhands@all-hands.dev>

Author: openhands-agent