Add tip to universal auth doc to show US/EU API URLs

docs/documentation/platform/identities/universal-auth.mdx

@@ -114,6 +114,13 @@ using the Universal Auth authentication method.
     that is to exchange the **Client ID** and **Client Secret** of the identity for an access token
     by making a request to the `/api/v1/auth/universal-auth/login` endpoint.
     
+    <Tip>
+      Choose the correct base URL based on your region:
+      
+      - For Infisical Cloud US users: `https://app.infisical.com`
+      - For Infisical Cloud EU users: `https://eu.infisical.com`
+    </Tip>
+
     #### Sample request
 
     ```bash Request