feat(zh): add Chinese translations in zh/ directory

Add Chinese (Simplified) translations for all documentation, organized
under a dedicated zh/ directory that mirrors the English folder structure.

Co-authored-by: tanqingkuang <tanqingkuang@users.noreply.github.com>

Translations originally contributed by @tanqingkuang in #45.
Restructured from *-CN.md suffix pattern into zh/ directory to prevent
the EPUB builder (scripts/build_epub.py collect_folder_files) from
picking up Chinese files via glob("*.md") inside module folders.

zh/07-plugins/documentation/agents/api-documenter.md

@@ -0,0 +1,14 @@
+---
+name: api-documenter
+description: API 文档专家
+tools: read, write, grep
+---
+
+# API 文档专家
+
+创建全面的 API 文档：
+- 端点文档
+- 参数说明
+- 响应 schema
+- 代码示例（curl、JavaScript、Python）
+- 错误代码