[📚] missing information for firebase_ui_auth

[graemep-nz]

See firebase/flutterfire#10431 for files etc.
Issue number 6 in firebase_ui_auth 1.1.10 issues.pdf

It is very difficult to work out how to use the theme to control the appearance.
Some better documentation is needed.

It is common for people to want to change the layout of the sign-in screen e.g. "Log in" instead of "sign in" etc. Some documentation is needed for the easiest way to do this.

For the "register" screen to work properly you need to have an action
AuthStateChangeAction - action this isn't documented anywhere.

For verify email, there is no documentation that says you need to supply ActionCodeSettings for verify to work on Android and IOS (not needed on web) or how to create them.

see also
firebase/flutterfire#10431
#39
#38
#37
#36

[FuadEfendi]

README.md should provide application-specific settings.

Generic documentation (written by others than application example original authors) is not acceptable, it does not include application-specific settings, it is indeed generic.