Update README with comprehensive JSON5 usage guide and remove placeholder content

[Copilot]

This PR completely updates the project README to provide comprehensive documentation for the JSON5 Kotlin library, replacing the outdated placeholder content that incorrectly claimed the project was "non-functional."

Changes Made

• Removed misleading warning: Eliminated the experimental/non-functional warning since the project has a working implementation with passing tests
• Added comprehensive project description: Clear explanation of JSON5 capabilities and backward compatibility with JSON
• Documented all JSON5 features: Comments, trailing commas, unquoted keys, single quotes, multi-line strings, extended number formats
• Provided practical usage examples:
  • Basic parsing with JSON5.parse() and stringifying with JSON5.stringify()
  • kotlinx.serialization integration with JSON5.encodeToString() and JSON5.decodeFromString()
  • Advanced features showcasing various JSON5 syntax extensions
• Added installation instructions: Proper dependency configuration for Gradle
• Simplified build section: Focused on library development rather than generic Gradle tutorial
• Replaced placeholder content: All generic multi-module project references removed

Before/After Comparison

Before: Generic template with warning about non-functionality and lengthy Gradle tutorials
After: Professional library documentation with clear API examples and feature explanations

The examples are based on the actual working API and test cases from the codebase, ensuring accuracy and usefulness for developers wanting to use this JSON5 library.

Fixes #8.

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