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/apps/resources/sidebar.mjs

@@ -2045,6 +2045,11 @@ export const sidebar = sidebarAttachHrefCommonOptions([
           },
         ],
       },
+      {
+        type: "link",
+        path: "/js-sdk",
+        title: "JS SDK",
+      },
       {
         type: "link",
         path: "/nextjs-starter",