docs: correct stale routing and url references

Document that the CLI mounts the content directory at the URL root and
that the app uses History API routing, not the old /content hash scheme.
Replace the removed <Article> iconPath prop with the actual upward
assets/ lookup, and fix the deck_preview preview_url example.

docs/mcp.md

@@ -246,10 +246,12 @@ ttrpg mcp generate-card-deck \
 {
   "success": true,
   "message": "预览文件已创建",
-  "preview_url": "http://localhost:3000/#/content/cards.md"
+  "preview_url": "http://localhost:3000/cards"
 }
 ```
 
+`preview_url` 以服务器工作目录为基准，去掉了 `.md` 扩展名（需要 `tttk serve` 正在运行）。
+
 ## CSV 文件格式
 
 CSV 文件使用 YAML frontmatter 定义模板和配置：