docs: redesign README and add contributor guidelines

[JasonXuDeveloper]

Summary

• Redesign README.md and README_zh_cn.md for better readability and quick value communication
• Add CONTRIBUTING.md with comprehensive contributor guidelines

Changes

README Enhancements (Both Languages)

• New logo: Cleaner socialify URL with JetBrains Mono font
• Removed: Repobeats analytics image, redundant # JEngine title
• Added: "Why JEngine?" section with scannable bullet list:
  • Lightweight design
  • One-click build with obfuscation
  • 10x+ performance vs ILRuntime/Lua
  • Zero/Minimal GC (UniTask + Nino)
  • All-platform support
  • Secure updates (DLL obfuscation + XOR/AES/ChaCha20 encryption)
  • Commercial readiness
• Reorganized packages:
  • Official Packages: JEngine.Core (Required), JEngine.Util (Optional)
  • Third-Party Packages: MetaJUI, MetaExcelDataTool
• Fixed: Chinese documentation link now shows as 中文文档
• Maintained: Release workflow markers intact

New CONTRIBUTING.md

Comprehensive contributor guide covering:

• Prerequisites (Unity 2022.3+, Git)
• Development setup instructions
• Project structure (clarified HotUpdate assets/code, Samples as dependency extensions)
• Code style guidelines (contributor's own name/email in file headers)
• Conventional Commits format with examples
• DCO sign-off requirement
• Pull request process
• CI/CD checks (Unity tests, DCO, CodeQL, Claude review)
• Community links (docs, QQ group)

Test plan

• Verify README.md renders correctly on GitHub
• Verify README_zh_cn.md renders correctly on GitHub
• Verify CONTRIBUTING.md renders correctly on GitHub
• Confirm release workflow awk commands still match markers (tested locally)

🤖 Generated with Claude Code

[claude]

Code Review

Found 1 issue that needs attention:

File Header Format Inconsistency

File: CONTRIBUTING.md, lines 99-100

The C# file header format shown in CONTRIBUTING.md differs from the format specified in CLAUDE.md.

CLAUDE.md specifies:

// FileName.cs
//
//  Author:
//        JasonXuDeveloper <jason@xgamedev.net>
//
//  Copyright (c) 2025 JEngine
//
//  [MIT License...]

CONTRIBUTING.md shows:

// FileName.cs
//
// Author: JasonXuDeveloper <jason@xgamedev.net>
// Copyright (c) 2025 JEngine - MIT License

Key differences:

• Missing indentation (CLAUDE.md has 2 spaces before "Author:" and 8 spaces before the email)
• Missing blank lines between sections
• Copyright and license are combined instead of separate lines

Impact: This inconsistency could confuse contributors and lead to non-uniform file headers across the codebase.

Recommendation: Update CONTRIBUTING.md lines 99-100 to match the CLAUDE.md format exactly.

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.