docs: add Node.js deploy guide and runnable example for Datum compute

[scotwells]

What this delivers

A self-contained guide that takes a developer from a Node.js HTTP service in source form to a live, reachable Instance on Datum compute, plus a runnable example they can deploy as-is. It lowers the barrier to a first successful Node.js deploy: package the app as a Unikraft unikernel on the base-compat runtime, publish to the metro registry, deploy with datumctl compute deploy, and verify the HTTP response end to end.

Changes

• Guide now lives at docs/guides/deploy-a-node-app.md (moved out of docs/compute/guides/).
• Ready-to-deploy example at examples/hello-node/ materializes every file the guide inlines, so a reader can clone and ship without copy-pasting:
  • app.js — the Node HTTP service (responds on / and /healthz)
  • package.json
  • Dockerfile — builds the minimal scratch rootfs with the node interpreter and its shared libraries
  • Kraftfile — base-compat:latest runtime packaging
  • workload.yaml — the Datum Workload manifest
  • README.md — one-line summary, link back to the guide, and the key build + deploy commands

The guide gains a single pointer line directing readers to the example folder.

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