[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).

Comienza con la configuración de pruebas de AngularFire 7 en torno a AngularFireModule y provideFirebaseApp, y luego reproduce el error de unit test reportado mientras simulas los métodos de Firestore, Auth y Analytics. Se considera completado cuando se documente una guía funcional para simular estos métodos de AngularFire con Angular 12, Firebase 9 y AngularFire 7.

Escrito por el modelo de indexación a partir del texto del issue.