docs: add index module to framework dropdown (medusajs#12748)

Author: shahednasser

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

@@ -34,6 +34,11 @@ export const navDropdownItems: NavigationItem[] = [
             title: "Events and Subscribers",
             link: "/learn/fundamentals/events-and-subscribers",
           },
+          {
+            type: "link",
+            title: "Index Module",
+            link: "/learn/fundamentals/module-links/index-module",
+          },
           {
             type: "link",
             title: "Medusa Container",