docs: add vitepress-plugin-mermaid for Mermaid diagram rendering

- 添加 vitepress-plugin-mermaid 和 mermaid 依赖
- 更新 VitePress 配置以支持 Mermaid 图表渲染
- 在 sidebar 中添加项目架构文档链接

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

package.json

@@ -5,6 +5,8 @@
     "docs:preview": "vitepress preview docs"
   },
   "devDependencies": {
-    "vitepress": "^1.3.4"
+    "mermaid": "^11.12.2",
+    "vitepress": "^1.3.4",
+    "vitepress-plugin-mermaid": "^2.0.17"
   }
 }