docs: use a Python image in the service expose example

[mpereira]

Summary

The service forwarding example in the sandbox overview runs python -m http.server without --from, so it uses the default workload image. That image is a minimal Ubuntu userspace with no Python, and the example fails. This PR makes the example select an image that includes Python.

Related Issue

I feel that this is small enough that it doesn't require an issue?

Changes

• Add --from docker.io/library/python:3.14-slim to the openshell sandbox create --expose example in docs/how-it-works/sandboxes/overview.mdx.
• Call python3 instead of python in the example command.
• Explain that the example uses a Python image because the built-in default workload image does not include Python, linking to the support matrix.

Testing

• Ran the updated example against a local gateway and opened the printed service URL
• mise run docs
• mise run pre-commit
• Unit tests added/updated (not applicable, docs only)
• E2E tests added/updated (not applicable, docs only)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (not applicable)

[copy-pr-bot]

This pull request requires additional validation before any workflows can run on NVIDIA's runners.

Pull request vetters can view their responsibilities here.

Contributors can view more details about this message here.

[github-actions]

All contributors have signed the DCO ✍️ ✅
_{Posted by the DCO Assistant Lite bot.}

[mpereira]

I have read the DCO document and I hereby sign the DCO.

[elezar]

This was caused by the switch to a basic noble-based image in #3386.

[elezar]

/ok-to-test 83e7678