docs(auth): add pointer to local MFA/TOTP example

could be useful to implementors

Author: mikehardy

docs/auth/multi-factor-auth.md

@@ -14,6 +14,8 @@ The [official guide for Firebase web TOTP authentication](https://firebase.googl
 
 The API details and usage examples may be combined with the full Phone auth example below to give you an MFA solution that fully supports TOTP or SMS MFA.
 
+You may also find it useful to investigate [the local / manual test screens](https://github.com/invertase/react-native-firebase/blob/main/tests/local-tests/auth/auth-totp-demonstrator.tsx) that we use to verify this functionality.
+
 # Phone MFA
 
 ## iOS Setup