docs: add PHP deploy guide and runnable example for Datum compute

[scotwells]

Summary

Adds a complete, product-focused guide for deploying a PHP web service on Datum compute, taking a developer from source code to a live, reachable Instance. Alongside the prose, this PR ships a ready-to-deploy example so users can clone and run it without copying snippets by hand.

What's included

• Guide at docs/guides/deploy-a-php-app.md — walks through writing the app, packaging it as a Unikraft unikernel image with kraft, publishing to the metro registry, deploying with datumctl compute deploy, verifying the running Instance, updating, and cleaning up. Includes a troubleshooting section covering the common boot, image-pull, and scheduling failure modes.
• Runnable example at examples/hello-php/ — the same files the guide describes, ready to deploy as-is:
  • server.php — the PHP HTTP service
  • Dockerfile — builds the trimmed shared-library closure the unikernel needs
  • Kraftfile — Unikraft Cloud packaging
  • workload.yaml — the Datum compute Workload manifest
  • README.md — one-line description, link back to the guide, and the key build + deploy commands

The guide links to the example, and the example links back to the guide, so a reader can move between the explanation and the working files in either direction.

🤖 Generated with Claude Code

[ecv]

so help me god if this ends with datum running PHP apps at massive scale