Enhanced GitHub Pages with comprehensive project overview and features

[Copilot]

This PR transforms the GitHub Pages static site from a simple benchmark visualization into a comprehensive project homepage that provides visitors with complete context about json5-kotlin while preserving all existing performance data.

Changes Made

🎨 Professional Design & Navigation

• Added beautiful gradient header with project branding
• Implemented tabbed navigation interface (Overview, Features, Quick Start, Benchmarks)
• Responsive design that works across all devices
• Modern UI with consistent styling throughout

📖 Comprehensive Content Sections

Overview Tab

• Project introduction and key benefits
• Feature highlights with modern card layout
• Call-to-action buttons linking to GitHub and documentation

Features Tab

• Detailed JSON5 features with syntax-highlighted code examples:
  • Comments (// and /* */)
  • Unquoted object keys
  • Trailing commas
  • Single quotes for strings
  • Multi-line strings
  • Extended number formats (hex, infinity, NaN)

Quick Start Tab

• Complete installation instructions with GitHub Packages setup
• Basic usage examples
• kotlinx.serialization integration examples
• Links to documentation and example applications

Benchmarks Tab

• All original benchmark functionality preserved exactly
• Interactive performance charts comparing JSON5 vs JSON vs External-JSON5
• Performance insights and methodology details

🔧 Technical Implementation

• Single-page application with JavaScript tab switching
• Charts load dynamically when benchmark tab is accessed
• Maintains all existing external dependencies (Chart.js CDN)
• Compatible with existing GitHub Pages workflow
• No breaking changes to build system

Before vs After

• Before: Simple benchmark page with limited project context
• After: Complete project homepage providing full overview + preserved benchmark data

The enhanced site now serves as both a project landing page and performance showcase, giving visitors comprehensive understanding of json5-kotlin's capabilities and performance characteristics.

Fixes #59.

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[hossain-khan]

Fantastic! 🎨