Move project docs to docs/project-docs and update references

- Move AGENTS.md, CLEANUP_SUMMARY.md, DOCUMENTATION_GUIDE.md,
  IMPLEMENTATION_SUMMARY.md, QUICK_COMMANDS.md to docs/project-docs/
- Update AGENTS.md to include splicing module documentation
- Update mkdocs.yml navigation to include project-docs section
- Update .gitignore to track docs/ directory
- Add docs/plans/ splicing design documents

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

mkdocs.yml

@@ -131,7 +131,7 @@ nav:
     - index.md
     - 快速开始: getting-started.md
     - 安装指南: installation.md
-  
+
   - 用户指南:
     - user-guide/index.md
     - MacrolactoneFragmenter 使用: user-guide/fragmenter-usage.md
@@ -139,14 +139,14 @@ nav:
     - 可视化功能: user-guide/visualization.md
     - 批量处理: user-guide/batch-processing.md
     - 数据导出: user-guide/data-export.md
-  
+
   - 教程与示例:
     - tutorials/index.md
     - 基础教程: tutorials/basic-tutorial.md
     - 环数识别教程: tutorials/using-macro-lactone-analyzer.md
     - 高级用法: tutorials/advanced-usage.md
     - 使用案例: tutorials/use-cases.md
-  
+
   - API 参考:
     - api/index.md
     - MacroLactoneAnalyzer: api/macro-lactone-analyzer.md
@@ -155,13 +155,20 @@ nav:
     - 环编号模块: api/ring-numbering.md
     - 可视化模块: api/ring-visualization.md
     - 工具函数: api/utilities.md
-  
+
   - 开发者指南:
     - development/index.md
     - 贡献指南: development/contributing.md
     - 项目结构: development/project-structure.md
     - 测试: development/testing.md
-  
+
+  - 项目文档:
+    - project-docs/AGENTS.md
+    - 实现总结: project-docs/IMPLEMENTATION_SUMMARY.md
+    - 清理总结: project-docs/CLEANUP_SUMMARY.md
+    - 文档指南: project-docs/DOCUMENTATION_GUIDE.md
+    - 快速命令: project-docs/QUICK_COMMANDS.md
+
   - 关于:
     - about/index.md
     - 更新日志: about/changelog.md