Comprehensive README documentation improvements

[Copilot]

This PR transforms the README.md from a basic, discouraging document into a comprehensive, user-friendly guide that properly showcases the calheatmapR package capabilities.

Major Improvements

Structure and Organization

• Complete restructure with logical sections: Overview, Installation, Data Format, Quick Start, Examples, API Reference, Troubleshooting
• Professional formatting with badges, emojis, and clear headings
• Better hierarchy that guides users from basic concepts to advanced usage

Content Enhancement

• Compelling package description highlighting key features and use cases instead of starting with deprecation warnings
• Detailed installation guide with prerequisites and step-by-step instructions
• Comprehensive data format explanation with multiple examples showing how to work with timestamps and convert existing data
• Complete API reference documenting all functions (calheatmapR, chDomain, chLegend, chLabel) with parameters

Practical Examples

• 6+ working code examples including:
  • Basic usage with simple data
  • GitHub-style contribution graphs
  • Business metrics dashboards
  • Using the included sample data
  • Complete Shiny integration example
• Real-world scenarios that users can adapt to their needs

User Experience

• Troubleshooting section with common issues and debugging code
• Domain/SubDomain reference table showing valid combinations
• Contributing guidelines encouraging community participation
• Balanced package status that acknowledges limitations while highlighting capabilities

Before vs After

• Length: 74 lines → 475 lines
• Tone: Discouraging → Welcoming and helpful
• Examples: 2 basic → 6+ comprehensive with explanations
• Coverage: Minimal → Complete API and usage documentation

The README now serves as a proper introduction and reference guide that helps users successfully install, understand, and use the calheatmapR package for creating calendar heatmaps in R.

Fixes #1.

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