Update checklists README with comprehensive documentation

[Copilot]

The existing README in the /checklists directory was minimal (only 5 lines) and lacked essential information for contributors. This update provides comprehensive documentation to help users understand and contribute to the Azure review checklists.

What's improved

📋 Overview and Structure

• Clear explanation of what's in the directory
• File naming conventions and patterns
• Available checklists and their maturity states (GA vs Preview)

🛠️ Step-by-step Checklist Creation Guide

• Complete walkthrough for creating new checklists
• JSON structure examples with real field names
• Schema requirements and validation rules
• GUID generation instructions

✏️ Modification Instructions

• Two options: direct JSON editing vs Excel spreadsheet
• Clear guidance on which files to modify (English only)
• Schema compliance requirements

🌍 Localization Guidelines

• Explanation of automated translation system
• Important warnings about not modifying translated files
• Why only English versions should be edited

🆘 Help and Support

• Multiple support channels with direct links to issue templates
• Links to contributing guidelines and documentation
• Community guidelines and quality assurance information

Key improvements

• Expanded from 5 lines to 166 lines with actionable content
• Step-by-step examples with actual JSON snippets
• Direct links to all referenced files and resources
• Clear warnings about localization to prevent common mistakes
• Multiple help channels for different types of issues

All referenced files and links have been verified to exist in the repository.

Fixes #1033.

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