docs: document admin environment variables (#11313)

www/apps/book/app/learn/customization/customize-admin/route/page.mdx

@@ -156,8 +156,8 @@ If you didn't follow the [previous chapter](../widget/page.mdx), create the file
 import Medusa from "@medusajs/js-sdk"
 
 export const sdk = new Medusa({
-  baseUrl: "http://localhost:9000",
-  debug: process.env.NODE_ENV === "development",
+  baseUrl: import.meta.env.VITE_BACKEND_URL || "/",
+  debug: import.meta.env.DEV,
   auth: {
     type: "session",
   },
@@ -170,6 +170,8 @@ You initialize the SDK passing it the following options:
 - `debug`: Whether to enable logging debug messages. This should only be enabled in development.
 - `auth.type`: The authentication method used in the client application, which is `session` in the Medusa Admin dashboard.
 
+Notice that you use `import.meta.env` to access environment variables in your customizations because the Medusa Admin is built on top of Vite. Learn more in [this chapter](../../../fundamentals/admin/environment-variables/page.mdx).
+
 You can now use the SDK to send requests to the Medusa server.
 
 <Note>