docs: add Firebase examples

[escamoteur]

@chimon2000 cloned issue escamoteur/flutter_command#6 on 2020-11-03:

It would be nice to have firebase examples using flutter_command. The two most common examples in the wild are login and todos.

[escamoteur]

@escamoteur commented on 2020-11-04:

Do you know any existing example that would be worth porting?