feature(wzn): add language switcher and complete some Chinese docs

[zunian-wan]

📋 Summary

Add language switcher for en & zh in LightRFT docs and complete Chinese documentation for model testing, design philosophy, troubleshooting, and core project architecture.

Purpose:

Type of Change:

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📚 Documentation update
• 🎨 Code refactoring (no functional changes)
• ⚡ Performance improvement
• ✅ Test addition/modification
• 🔧 Configuration/Build changes

🔗 Related Issues

None

📝 Changes

What changed:

Why these changes:

Key implementation details:

🧪 Testing

Test Plan

• Unit tests: Added/updated unit tests
• Integration tests: Tested with full training pipeline
• Manual testing: Describe what you tested manually

Test commands:

# Commands used to test the changes

Test Results

Test Output

Paste test output here

📊 Performance Impact

• No performance impact
• Performance improved:
• Performance regression:

📚 Documentation

• Docstrings updated for new/modified functions
• README.md updated (if user-facing changes)
• Documentation in docs/ updated (if applicable)
• Examples updated/added (if applicable)
• Configuration reference updated (if new parameters added)
• CHANGELOG.md updated

✅ Checklist

Code Quality

• Code follows the project's style guidelines (run make format and make fcheck)
• Self-review of code completed
• Code is well-commented, especially in complex areas
• No unnecessary debug logs or commented-out code

Compatibility

• Changes are backward compatible (or breaking changes are documented)
• Existing tests pass with changes
• No new warnings introduced

Testing

• Tested with FSDP (if applicable)
• Tested with DeepSpeed (if applicable)
• Tested with inference engines (vLLM/SGLang) (if applicable)
• Tested on multiple GPU configurations (if applicable)

Documentation

• All public APIs are documented
• User-facing changes are documented
• Migration guide provided (if breaking changes)

💭 Additional Notes

🔍 Review Checklist for Maintainers

• Code quality and style verified
• Tests are adequate and passing
• Documentation is complete and clear
• Performance impact is acceptable
• Breaking changes are properly documented
• Ready to merge

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• Contributing Guide
• Code Style Guidelines