v1.5.3 - guides.xml modernization

Changes

• Prefer guides.xml (modern PHP-based rendering) over Settings.cfg (legacy Sphinx-based)
• Add webhook status codes to intercept-deployment.md:
  • 200 - ping success
  • 204 - push accepted
  • 412 - precondition failed (expected on first-time)
• Add GitHub CLI automation examples for webhook setup
• Fix validate_docs.sh to accept either guides.xml OR Settings.cfg
• Update templates/AGENTS.md with complete guides.xml configuration example
• Update typo3-extension-architecture.md documentation requirements

Migration Note

New extensions should use guides.xml (modern PHP-based rendering). Existing extensions with Settings.cfg should consider migrating to guides.xml for improved rendering capabilities.