docs: add admin extensions to navbar (#14368)

www/packages/docs-ui/src/constants.tsx

@@ -76,6 +76,11 @@ export const navDropdownItems: NavigationItem[] = [
           },
         ],
       },
+      {
+        type: "link",
+        title: "Admin Extensions",
+        link: "/learn/fundamentals/admin",
+      },
       {
         type: "sub-menu",
         title: "Commerce Modules",