feat: Add command documentation dialog

Introduce a modal dialog that displays documentation for all available
markdown commands, accessible via a '?' button in the header.

src/components/index.ts

@@ -18,6 +18,8 @@ export type { ArticleProps } from "./Article";
 export { MobileSidebar, DesktopSidebar } from "./Sidebar";
 export type { SidebarProps } from "./Sidebar";
 export { FileTreeNode, HeadingNode } from "./FileTree";
+export { default as DocDialog } from "./DocDialog";
+export type { DocDialogProps } from "./DocDialog";
 
 // 导出数据类型
 export type { DiceProps } from "./md-dice";