[docs] Mocking of angularfire methods with angularfire 7 during tests

### Version info

**Angular:** 12

**Firebase:** 9

**AngularFire:** 7

### How to reproduce these conditions

How to mock firebase operator during tests suits ?
How to write unit tests with mocks of firestore/auth/analytics methods ?

### Expected behavior

With Angularfire 6, I was able to mock angularfire methods like doc, collection and other methods to write my unit tests.
I would like to be able to mock it.

### Actual behavior

I am getting the error during my unit tests :
Error: Either AngularFireModule has not been provided in your AppModule (this can be done manually or implictly using provideFirebaseApp) or you're calling an AngularFire method outside of an NgModule (which is not supported).

Beginne mit dem AngularFire 7-Test-Setup rund um AngularFireModule und provideFirebaseApp und reproduziere anschließend den gemeldeten Unit-Test-Fehler, während du Firestore-, Auth- und Analytics-Methoden mockst. Als erledigt gilt die Dokumentation einer funktionierenden Anleitung zum Mocking dieser AngularFire-Methoden unter Angular 12, Firebase 9 und AngularFire 7.

Vom Indexierungsmodell aus dem Issue-Text verfasst.