feat(docs): restructure appendix content into organized directories

- Move standalone AI-related files into 8-artificial-intelligence directory
- Move development tools content into 2-development-tools directory
- Move server/backend content into 4-server-and-backend directory
- Create new index files for each section
- Update .gitignore to exclude old backup directories
- Update theme imports for new component locations

docs/zh-cn/appendix/9-engineering-excellence/code-quality-refactoring.md

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+# 代码质量与重构
+
+> 待实现

docs/zh-cn/appendix/9-engineering-excellence/design-patterns.md

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+# 设计模式
+
+> 待实现

docs/zh-cn/appendix/9-engineering-excellence/open-source-collaboration.md

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+# 开源协作
+
+> 待实现

docs/zh-cn/appendix/9-engineering-excellence/security-thinking.md

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+# 安全思维与攻防基础
+
+> 待实现

docs/zh-cn/appendix/9-engineering-excellence/technical-writing.md

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+# 技术文档写作
+
+> 待实现

docs/zh-cn/appendix/9-engineering-excellence/technology-selection.md

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+# 技术选型方法论
+
+> 待实现

docs/zh-cn/appendix/9-engineering-excellence/testing-strategies.md

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+# 测试策略
+
+> 待实现