Project cleanup and comprehensive documentation improvements

[Copilot]

This PR addresses comprehensive project cleanup and documentation improvements to make the json5-kotlin library more accessible and easier to understand for contributors and users.

Key Improvements

Documentation Enhancements

• Enhanced README.md with proper error handling examples, improved installation instructions, and comprehensive troubleshooting section
• Fixed API documentation throughout all guides to match the actual implementation (JSON5.parse() instead of outdated references)
• Improved usage examples with practical error handling using JSON5Exception and SerializationException
• Added GitHub Packages authentication guidance with notes about classic vs fine-grained tokens

Code Examples & Testing

• Corrected all code samples to use the proper JSON5 API methods
• Enhanced CONTRIBUTING.md with accurate test examples and debugging information
• Improved PR template with comprehensive testing checklist
• Added app module documentation explaining the demo application and sample JSON5 files

Project Configuration

• Updated outdated references (Gradle version in settings.gradle.kts)
• Enhanced .gitignore with comprehensive coverage for editors, OS files, and temporary files
• Improved benchmark documentation with better performance guidance and interpretation of results

Build & Development

• Enhanced development guides with correct API usage and debugging examples
• Added troubleshooting sections for common issues like Java version mismatches and GitHub Packages authentication
• Verified all builds, tests, and formatting work correctly after changes

Before/After Examples

Before:

val result = JSON5.parseToJsonElement(json5Text) // Incorrect API

After:

try {
    val result = JSON5.parse(json5Text)
    // Handle result...
} catch (e: JSON5Exception) {
    println("JSON5 parsing error: ${e.message}")
}

The documentation now provides accurate, practical examples with proper error handling that users can copy and use directly in their projects.

Testing

• ✅ All builds pass (./gradlew build)
• ✅ All checks pass (./gradlew check)
• ✅ Code formatting verified (./gradlew formatKotlin)
• ✅ Demo app runs successfully (./gradlew :app:run)
• ✅ All documentation examples validated against actual implementation

Fixes #64.

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.