docs: add section describing ProxiedSignInPage

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

docs/auth/google/gcp-iap-auth.md

@@ -91,23 +91,8 @@ sign-in mechanism to poll that endpoint through the IAP, on the user's behalf.
 
 ## Frontend Changes
 
-All Backstage apps need a `SignInPage` to be configured. Its purpose is to
-establish who the user is and what their identifying credentials are, blocking
-rendering the rest of the UI until that's complete, and then keeping those
-credentials fresh.
-
-When using IAP Proxy authentication, the Backstage UI will only be loaded once
-the user has already successfully completely authenticated themselves with the
-IAP and has an active session, so we don't want to make the user have to go
-through a _second_ layer of authentication flows after that.
-
-As such, we want to not display a sign-in page visually at all. Instead, we will
-pick a `SignInPage` implementation component which knows how to silently make
-requests to the backend provider we configured above, and just trusting its
-output to properly represent the current user. Luckily, Backstage comes with a
-component for this purpose out of the box.
-
-Update your `createApp` call in `packages/app/src/App.tsx`, as follows.
+It is recommended to use the `ProxiedSignInPage` for this provider, which is
+installed in `packages/app/src/App.tsx` like this:
 
 ```diff
 +import { ProxiedSignInPage } from '@backstage/core-components';
@@ -117,5 +102,4 @@ Update your `createApp` call in `packages/app/src/App.tsx`, as follows.
 +    SignInPage: props => <ProxiedSignInPage {...props} provider="gcp-iap" />,
 ```
 
-After this, your app should be ready to leverage the Identity-Aware Proxy for
-authentication!
+See the [Sign-In with Proxy Providers](../index.md#sign-in-with-proxy-providers) section for more information.