Improve Documentation for Amplify to trigger CUP OAuth2 flows for federated users

Request for documentation: How to use Amplify to trigger CUP OAuth2 flows for federated users.

The only related page is -> https://docs.amplify.aws/lib/auth/advanced/q/platform/js/

This page can be confusing:
* It brings in concept of Identity Pools federation - that is not relevant for a plain vanilla CUP customer,
* It talks about Google, Facbeook, social federation and tokens etc that are not relevant for a customer just wanting to send a user to CUP Oauth2 endpoints with PKCE.

This should contain a single page dedicated to this simple use-case. Folks can get confused by all the code here, give up and go looking for a PKCE library, or try to hand craft the OAuth2 redirect without PKCE.

The reality is that you just need maybe three lines of code:

document.getElementById("OAuthSignInButtonGoogle").addEventListener("click", (evt) => {
Auth.federatedSignIn({provider: 'Google'});
});

Further, the doc of the actual method of Auth is misleading:
https://aws-amplify.github.io/amplify-js/api/classes/authclass.html#federatedsignin

The signature talks of "LegacyProvider" - just by the name this sounds like something customers should not use. Please add text here to clarify what this is. The value can be google | string. Clarify what the string is = name of IdP in CUP.