docs: Organize documentation into structured folders

Organized documentation into cleaner structure:

Root directory (user-facing):
- README.md - Main documentation
- CLAUDE.md - AI context (referenced by system)
- QUICKSTART.md - Quick start guide

docs/ (technical documentation):
- CASL_AUTHORIZATION.md - Authorization guide
- ERROR_HANDLING.md - Error handling patterns
- REQUIREMENTS.md - Project requirements

docs/deployment/ (production deployment):
- HTTPS_SETUP.md - SSL/TLS setup
- PRODUCTION_ENVIRONMENT_TEMPLATE.md - Env vars template
- PRODUCTION_VERIFICATION_CHECKLIST.md - Deployment checklist

Removed:
- DOCKER_TROUBLESHOOTING.md - Outdated (referenced Google OAuth, old domain)

Updated references:
- Fixed links to moved files in CASL_AUTHORIZATION.md
- Fixed links to moved files in ERROR_HANDLING.md
- Removed reference to deleted BUILD_STATUS.md in QUICKSTART.md

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

QUICKSTART.md

@@ -230,7 +230,7 @@ Ready to deploy:
 **Need Help?**
 
 - Check [CLAUDE.md](CLAUDE.md) for comprehensive documentation
-- Check [BUILD_STATUS.md](BUILD_STATUS.md) for what's implemented
+- Check [README.md](README.md) for detailed feature overview
 - Check [backend/README.md](backend/README.md) for API docs
 - Check [frontend/README.md](frontend/README.md) for frontend docs