[📚] Add documentation on how to use Isolates with Firestore

[Lyokone]

https://github.com/firebase/flutterfire/issues/3124 https://github.com/firebase/flutterfire/issues/4846 https://github.com/firebase/flutterfire/issues/4129

[Ehesp]

Where would this documentation exist? If it's on the main site we'll probably need input from the Firebase team on where/how this should be documented

[Lyokone]

I would see it in the Firestore read documentation as a way to handle big documents. I'm not sure it's worth adding it directly to the cloud_firestore package.