docs(auth): adding 3rd party UI lib mentions to the docs (angular#2025)

Author: AnthonyNahas

docs/auth/getting-started.md

@@ -38,6 +38,11 @@ export class AppComponent {
 }
 ```
 
+## UI Libraries
+
+- Material Design : [ngx-auth-firebaseui](https://github.com/AnthonyNahas/ngx-auth-firebaseui)
+- Bootstrap : [@firebaseui/ng-bootstrap](https://github.com/firebaseui/ng-bootstrap)
+
 ## Cordova
 
 Learn how to [setup Firebase Authentication with Cordova](https://firebase.google.com/docs/auth/web/cordova) in the Firebase Guides.