doc: social logins signups redirect loop on android when android:taskAffinity=""

[wildsylvan]

when creating a new flutter app, the default template puts android:taskAffinity="" in android\app\src\main\AndroidManifest.xml and that makes registering a user through social login (e.g. google) not work correctly. basically the account is registered successfully in logto but logto doesn't redirect to the app, instead loops back to the google sign up screen:

(here the user is correctly registered in logto but then logto redirects to the google sign up confirmation again)

removing android:taskAffinity="" fixes the issue

[wildsylvan]

see: flutter/flutter#63559

[simeng-li]

#76