Create comprehensive developer documentation for new team onboarding

[Copilot]

This PR creates comprehensive developer documentation to support the onboarding of new development teams as requested in the issue. The documentation includes detailed architecture analysis, security recommendations, and development roadmap for the ImageTransform library.

Documentation Added

📋 INTRO.md

• Concise service overview for inclusion in Time Out's service catalog
• Clear description of the library's purpose and core capabilities
• Highlights dual backend support (PHP GD and ImageMagick)

🏗️ ARCHITECTURE.md

• Complete architecture breakdown with visual Mermaid diagrams
• C4 Context and Container diagrams showing system relationships and dependencies
• Detailed component analysis including:
  • Transformation orchestration layer
  • Image abstraction with GD/ImageMagick implementations
  • File I/O architecture and security considerations
  • Extension points for custom transformations
• Technology stack documentation covering PHP requirements, dependencies, and build tools
• Performance guidelines for backend selection and memory management
• API reference with usage patterns and integration examples
• Security considerations and error handling strategies

🛣️ ROADMAP.md

• Critical security improvements with risk assessments and implementation priorities
• Development experience modernization including:
  • PHP 8+ compatibility upgrades
  • Modern testing infrastructure with updated PHPUnit
  • CI/CD pipeline implementation
  • Enhanced developer tooling
• Technical debt resolution plan with architecture cleanup recommendations
• Implementation timeline structured in phases with resource requirements
• Quality assurance strategy including testing, monitoring, and success metrics
• Risk mitigation plans for breaking changes and security vulnerabilities

Key Features

• Visual architecture diagrams using Mermaid for clear system understanding
• Security-focused recommendations addressing input validation, memory management, and access controls
• Modernization roadmap to bring the PHP 5.3+ codebase up to current standards
• Practical guidance for extending the library with custom transformations
• Performance optimization strategies for different use cases

This documentation provides new developers with everything needed to understand, maintain, and extend the ImageTransform library effectively.

Fixes #3.

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