chore: set the bookmark script to English locale (#580)

* chore: set the bookmark script to English locale

* chore: adapt the booklet to the locale

* refactor(demo): improve naming to better distinguish mirror and lang

---------

Co-authored-by: Simon <10131203+gaomeng1900@users.noreply.github.com>

docs/developer-guide.md

@@ -91,7 +91,7 @@ npm run build:ext
 - Add a new bookmark
 
     ```javascript
-    javascript:(function(){var s=document.createElement('script');s.src=`http://localhost:5174/page-agent.demo.js?t=${Math.random()}`;s.onload=()=>console.log(%27PageAgent ready!%27);document.head.appendChild(s);})();
+    javascript:(function(){var s=document.createElement('script');s.src=`http://localhost:5174/page-agent.demo.js?lang=en-US&t=${Math.random()}`;s.onload=()=>console.log(%27PageAgent ready!%27);document.head.appendChild(s);})();
     ```
 
 - Click the bookmark on any page to load Page-Agent