docs: added troubleshooting guides + improvements (#11927)

* docs: added troubleshooting guides + improvements

* build fixes

www/apps/book/app/learn/fundamentals/admin/page.mdx

@@ -11,6 +11,8 @@ You can customize the admin dashboard by:
 - Adding new sections to existing pages using Widgets.
 - Adding new pages using UI Routes.
 
+However, you can't customize the admin dashboard's layout, design, or the content of the existing pages (aside from injecting widgets).
+
 ---
 
 ## Medusa UI Package