docs: enhance documentation with architecture, quickstart guide and modern formatting

[tekenstam]

Documentation Modernization and Repository Structure Improvements

Summary

This PR significantly enhances the documentation and repository structure for both alert-manager and iam-manager projects, making them more accessible to new users and contributors. The changes focus on improving readability, adding comprehensive guides, and following modern open source project best practices.

Changes

Documentation Improvements

• Added detailed architecture documentation with modern Mermaid diagrams that render directly in GitHub
• Created comprehensive quickstart guides with clear installation steps and examples
• Added detailed configuration documentation explaining all available options
• Developed troubleshooting guides with common issues and solutions
• Enhanced developer documentation with setup instructions and workflow guidance
• Added AWS integration documentation including IRSA setup
• Created a documentation landing page (docs/README.md) for better navigation

Repository Structure

• Moved CONTRIBUTING.md from .github/ to repository root for better visibility
• Completely revamped contributing guidelines with comprehensive information
• Added DCO signing requirements with explanation and instructions
• Normalized naming conventions for documentation files

README Enhancements

• Added a Table of Contents for easier navigation
• Improved organization with clear section headings
• Enhanced descriptions of components and features
• Added Version Compatibility tables with accurate information
• Updated and fixed badge references
• Added links to the new documentation

Technical Documentation

• Added specific information about Kubebuilder usage and patterns
• Created modernized architecture diagrams using Mermaid for better maintainability
• Improved explanation of AWS integration and security concepts

Testing

• Verified all documentation links work correctly
• Confirmed Mermaid diagrams render properly in GitHub
• Ensured references to renamed files are updated