Set up GitHub Cloudflare publishing flow and add SVG favicon

[jillesvangurp]

Summary

• add GitHub Actions PR validation workflow for Hugo build on pull requests to main
• add GitHub Actions deployment workflow to publish to Cloudflare Pages on pushes to main
• update README to document the GitHub publishing flow and required repository secrets
• use the existing Open RTLS logo SVG as the site favicon in the shared base template

Validation

• npm run build