message: docs/auth: add proxied sign-in page selection
diff: @@ -139,15 +139,51 @@ All the sign-in page needs to do is to call the `/refresh` endpoint of the auth
  to get the existing session, which is exactly what the `ProxiedSignInPage` does. The only
  thing you need to do to configure the `ProxiedSignInPage` is to pass the ID of the provider like this:
  -```diff
  +```tsx
  const app = createApp({
  - apis,
  -+ components: {
  -+ SignInPage: props => <ProxiedSignInPage {...props} provider="awsalb" />,
  -+ },
  - bindRoutes({ bind }) {
  + ...,
  + components: {
  + SignInPage: props => <ProxiedSignInPage {...props} provider="awsalb" />,
  + },
  +});
  ```
  +A downside of this method is that it can be cumbersome to set up for local development.
  +As a workaround for this, it's possible to dynamically select the sign-in page based on
  +what environment the app is running in, and then use a different sign-in method for local
  +development, if one is needed at all. Depending on the exact setup, one might choose to
  +select the sign-in method based on the `process.env.NODE_ENV` environment variable,
  +by checking the `hostname` of the current location, or by accessing the configuration API
  +to read a configuration value. For example:
  +
  +```tsx
  +const app = createApp({
  + ...,
  + components: {
  + SignInPage: props => {
  + const configApi = useApi(configApiRef);
  + if (configApi.getString('auth.environment') === 'development') {
  + return (
  + <SignInPage
  + {...props}
  + provider={{
  + id: 'google-auth-provider',
  + title: 'Google',
  + message: 'Sign In using Google',
  + apiRef: googleAuthApiRef,
  + }}
  + />
  + );
  + }
  + return <ProxiedSignInPage {...props} provider="gcpiap" />;
  + },
  + },
  +});
  +```
  +
  +When using multiple auth providers like this, it's important that you configure the different
  +sign-in resolvers so that they resolve to the same identity regardless of the method used.
  +
  ## For Plugin Developers
  The Backstage frontend core APIs provide a set of Utility APIs for plugin developers
  