docs: add railway troubleshooting for host (#11346)

www/apps/book/app/learn/fundamentals/admin/environment-variables/page.mdx

@@ -6,6 +6,12 @@ export const metadata = {
 
 In this chapter, you'll learn how to use environment variables in your admin customizations.
 
+<Note>
+
+To learn how envirnment variables are generally loaded in Medusa based on your application's environment, check out [this chapter](../../environment-variables/page.mdx).
+
+</Note>
+
 ## How to Set Environment Variables
 
 The Medusa Admin is built on top of [Vite](https://vite.dev/). To set an environment variable that you want to use in a widget or UI route, prefix the environment variable with `VITE_`.