Docs: surface Codespaces / Dev Containers in README (follow-up to #27)

[achandmsft]

Follow-up to #27 (dev container).

The dev container is in — make sure folks actually find it from the README:

• Codespaces + Dev Containers badges under the short link at the top of the README, so the one-click try is the first thing visitors see after the title.
• A blurb under the badges explaining what they get (preinstalled az, azd, Python 3.13) with a deep link to the new section.
• Prerequisites now ends with "Or just open the repo in GitHub Codespaces / VS Code Dev Containers" — people who hit the "I don't want to install all that" reaction at the prerequisites list get an immediate out.
• Quickstart has a one-liner pointing at the Codespaces variant.
• New Quickstart in Codespaces subsection (just before Configuration) with the full four-step flow: badge → az login --use-device-code (required because the browser flow doesn't work the same inside a codespace) → azd env new → azd up. Plus a note that the same dev container works locally via the Dev Containers extension.

No code changes — pure docs.