chore(ui): add migration guide (#9733)

* chore(ui_auth): add migration guide

* chore(ui_auth): add firestore and datatable sections

Author: lesnitsky

Diff for: packages/firebase_ui_auth/doc/migration.md

@@ -0,0 +1,122 @@
+# FlutterFireUI ➡️ FirebaseUI migration guide
+
+To migrate from `flutterfire_ui` package to `firebase_ui_*` family, you need to do the following:
+
+## Updating dependencies
+
+For Firebase Auth widgets:
+
+```diff
+dependencies:
+-  flutterfire_ui: ^0.4.0
++  firebase_ui_auth: ^1.0.0
+```
+
+If you're using OAuth providers:
+
+```diff
+dependencies:
+# ...
++  firebase_ui_oauth: ^1.0.0
++  firebase_ui_oauth_google: ^1.0.0
++  firebase_ui_oauth_apple: ^1.0.0
+```
+
+Please make sure to depend only on those providers that are actually being used in your app. Having the provider included, but not configured might lead to unexpected behaviour.
+
+All supported OAuth providers:
+
+- [firebase_ui_oauth_apple](https://pub.dev/packages/firebase_ui_oauth_apple)
+- [firebase_ui_oauth_facebook](https://pub.dev/packages/firebase_ui_oauth_facebook)
+- [firebase_ui_oauth_google](https://pub.dev/packages/firebase_ui_oauth_google)
+- [firebase_ui_oauth_twitter](https://pub.dev/packages/firebase_ui_oauth_twitter)
+
+Make sure to update your imports as well:
+
+```diff
+- import 'package:flutterfire_ui/auth.dart';
++ import 'package:firebase_ui_auth/firebase_ui_auth.dart';
+```
+
+If you're using OAuth providers, you need to import those from corresponding packages:
+
+```diff
+// All OAuth providers used to be under flutterfire_ui
+- import 'package:flutterfire_ui/auth.dart';
++ import 'package:firebase_ui_oauth_google/firebase_ui_oauth_google.dart';
+```
+
+## Code adjustments
+
+Below is the list of necessary changes that you have to make to migrate to `firebase_ui_*` packages.
+
+### Configuration
+
+- `FlutterFireUIAuth` was renamed to `FirebaseUIAuth`
+- instead of passing an instance of `<provider name>ProviderConfiguration` you need to pass an instance of the `<provider name>Provider`
+
+```diff
+- FlutterFireUIAuth.configureProviders([
+-    const EmailProviderConfiguration(),
+- ]);
+
++ FirebaseUIAuth.configureProviders([
++    const EmailProvider(),
++ ]);
+```
+
+## Sign out
+
+```diff
+- await FlutterFireUIAuth.signOut();
++ await FirebaseUIAuth.signOut();
+```
+
+## Profile screen
+
+If you're using `ProfileScreen` – make sure to add the following to your `pubspec.yaml`:
+
+```yaml
+fonts:
+  - family: SocialIcons
+    fonts:
+      - asset: packages/firebase_ui_auth/fonts/SocialIcons.ttf
+```
+
+## Migrating to `firebase_ui_firestore`
+
+To migrate from `flutterfire_ui` to `firebase_ui_firestore` you need to update your dependencies:
+
+```diff
+dependencies:
+-  flutterfire_ui: ^0.4.0
++  firebase_ui_firestore: ^1.0.0
+```
+
+and imports:
+
+```diff
+- import 'package:flutterfire_ui/firestore.dart';
++ import 'package:firebase_ui_firestore/firebase_ui_firestore.dart';
+```
+
+## Migrating to `firebase_ui_database`
+
+To migrate from `flutterfire_ui` to `firebase_ui_database` you need to update your dependencies:
+
+```diff
+dependencies:
+-  flutterfire_ui: ^0.4.0
++  firebase_ui_database: ^1.0.0
+```
+
+and imports:
+
+```diff
+- import 'package:flutterfire_ui/database.dart';
++ import 'package:firebase_ui_database/firebase_ui_database.dart';
+```
+
+---
+
+> Check out [full documentation](./README.md) for more details.