Add Deno support to GitHub Pages template

This pull request enhances the existing Astro GitHub Pages template by adding first-class support for building under Deno, in line with the upstream changes in [withastro/action#77](https://github.com/withastro/action/pull/77).

📝 What’s Changed

1. Documentation

• Added a “🔧 Enabling Deno Support” section with:
  • Add deno.json configuration
  • Explanation of the Deno task setup & build flow
  • Updated the project structure diagram to surface deno.json as an optional root file.

2. Deno Configuration (deno.json)

• Introduced a minimal example that:
  • Enables Deno’s Node-builtins compatibility
    • Defines a setup task (caching) and a build task (npm:astro build via Deno’s loader)

3. GitHub Actions Workflow

🎯 Motivation

• Leverage the new Deno support in withastro/action so that users who prefer Deno can build & deploy their Astro sites without installing Node.

• Keep the Node-first flow fully supported and unchanged for existing users.

🔗 References

• Upstream Deno support in action: Support Deno action#77
• Deployment guide: https://docs.astro.build/en/guides/deploy/github/

✅ Checklist

• README updated with clear Deno instructions
• Workflow file updated and tested on a sample Deno repo
• Backward compatibility verified for Node-only projects