docs: document JS SDK installation (#9611)

- Add a page introducing JS SDK + how to install and use it (generally)
- Adjust admin tips on how to send requests
- Adjust storefront tips to mention JS SDK
- Add in the API reference intro how to install JS SDK
- Other related additions / changes

Closes DX-957

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

@@ -76,6 +76,11 @@ export const navDropdownItems: NavigationItem[] = [
         title: "Medusa CLI",
         link: "/v2/resources/medusa-cli",
       },
+      {
+        type: "link",
+        title: "JS SDK",
+        link: "/v2/resources/js-sdk",
+      },
       {
         type: "link",
         title: "Next.js Starter",