docs: describe cache in Flutter Firebase Login Tutorial

Description

Clearly describe what documentation you are looking to add or improve. In https://bloclibrary.dev/#/flutterfirebaselogintutorial you guys just use cache, but never talk about cache, have nothing about it, if u didn't go in the repo in just CTRL+C, CTRL+V the files u can't do the tutorial by your own. even if you know about cache, why have no explanation or the code of cache in the tutorial, it makes no sense. "But it's a BLoC tutorial". Ok, but how can i follow if the tutorial it's incomplete. why you use things without explaining the propose, what this doing, why create this package... things like that