docs: various improvements to introduction guides (#14398)

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

@@ -71,11 +71,12 @@ To fetch data with a route loader:
 For example, consider the following UI route created at `src/admin/routes/custom/page.tsx`:
 
 export const loaderHighlights = [
-  ["6", "loader", "Export a loader function to fetch products."],
-  ["15", "useLoaderData", "Access the data returned by the loader function in the route component."],
+  ["7", "loader", "Export a loader function to fetch products."],
+  ["16", "useLoaderData", "Access the data returned by the loader function in the route component."],
 ]
 
 ```tsx title="src/admin/routes/custom/page.tsx" highlights={loaderHighlights}
+import { defineRouteConfig } from "@medusajs/admin-sdk"
 import { Container, Heading } from "@medusajs/ui"
 import {
   useLoaderData,

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

@@ -42,6 +42,7 @@ export const sdk = new Medusa({
 import Medusa from "@medusajs/js-sdk"
 
 export const sdk = new Medusa({
+  // add __BACKEND_URL__ to src/admin/vite-env.d.ts if you get type errors
   baseUrl: __BACKEND_URL__ || "/",
   auth: {
     type: "session",