chore: add callout with link to external IdP configuration in signInWithRedirect docs

Author: calebpollman

src/pages/[platform]/build-a-backend/auth/connect-your-frontend/sign-in/index.mdx

@@ -812,6 +812,12 @@ func confirmSignIn() -> AnyCancellable {
 
 To sign in using an external identity provider such as Google, use the `signInWithRedirect` function.
 
+<Callout info>
+
+For guidance on configuring an external Identity Provider with Amplify see [External Identity Providers](https://docs.amplify.aws/react/build-a-backend/auth/concepts/external-identity-providers/)
+
+</Callout>
+
 ```ts
 import { signInWithRedirect } from "aws-amplify/auth"