docs: add CONTRIBUTING.md with contribution guidelines

[afurm]

Summary

Adds a comprehensive CONTRIBUTING.md file to improve open source friendliness, addressing issue #64.

What

• CONTRIBUTING.md (285 lines) with:
  • Code style guide (Python 3.11+, 4-space indentation, naming conventions)
  • Branch strategy (feat/fix/docs/refactor/test prefixes)
  • Testing requirements (unittest framework, pytest pattern, integration test skip pattern)
  • Commit message format (type(scope): description)
  • PR description template with required sections
  • Issue guidelines (Bug Report, Feature Request, Question templates)
  • Bilingual EN/CN format matching project style

Why

Issue #64 identified that the project lacked contribution guidelines, making it harder for external contributors to participate. This CONTRIBUTING.md provides:

1. Clear code style expectations
2. Branch naming conventions
3. Testing requirements and patterns
4. Commit message format
5. PR description template
6. Issue templates for better categorization

The bilingual format (English/Chinese) matches the project's existing documentation style.

Testing

• File created and committed
• Branch pushed to fork
• Syntax validation: (not applicable)

Notes

• Follows project's bilingual documentation style
• Compatible with project's existing unittest-based testing approach
• Does not introduce any CI requirements (as per issue 开源贡献友好度建议：CONTRIBUTING.md / Issue 模板 / CI / etc #64's conservative approach)