docs: update social_authentication.md (#1105)

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## Description

Add paragraphs on `social_authentication.md` to specify all the
environment variables and user defined settings (Xcode) that the users
have to fill once finished the configuration of a specific social
authentication method. Updated Email sign in, Facebook and Twitter
methods.

Fixes flutter/flutter#140463

<!--- Describe your changes in detail -->

## Type of Change

<!--- Put an `x` in all the boxes that apply: -->

- [ ] ✨ New feature (non-breaking change which adds functionality)
- [ ] 🛠️ Bug fix (non-breaking change which fixes an issue)
- [ ] ❌ Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] 🧹 Code refactor
- [ ] ✅ Build configuration change
- [x] 📝 Documentation
- [ ] 🗑️ Chore

Author: valentinallavayol

docs/docs/project_configuration/social_authentication.md

@@ -9,7 +9,7 @@ The Flutter News Toolkit comes pre-configured to support authentication using pa
 
 ## Email
 
-The news toolkit supports passwordless login.
+The news toolkit supports passwordless login. This means a deep link is sent to the user's email address, that when clicked will open your app and log the user in.
 
 ### Firebase configuration
 
@@ -21,6 +21,12 @@ Passwordless authentication with an email link requires additional configuration
 
 :::
 
+Once the email authentication method is set up, go to **Firebase -> Engage -> Dynamic links**. Set up a new dynamic link URL prefix (for example, yourApplicationName.page.link) with a dynamic link URL of "/email_login".
+
+Once the dynamic link is set up, replace the placeholder value for **FLAVOR_DEEP_LINK_DOMAIN** inside the `launch.json` file with the **dynamic link URL prefix** you just created. This enviroment variable will be used inside `firebase_authentication_client.dart` to generate the dynamic link URL that will be sent to the user.
+
+In addition, replace the placeholder value for every **FLAVOR_DEEP_LINK_DOMAIN** key within your `project.pbxproj` file with the dynamic link URL prefix you just created.
+
 ## Google
 
 ### Firebase configuration
@@ -49,7 +55,11 @@ After setting up your [Firebase project](https://flutter.github.io/news_toolkit/
 
 ### Complete the setup
 
-To complete your setup, add the OAuth redirect URI listed in your **Firebase Authentication Sign-in Method** to your Facebook app configuration. For additional details, check out the [Firebase authentication](https://firebase.google.com/docs/auth/?authuser=0) page.
+To complete your setup, add the OAuth redirect URI listed in your **Firebase Authentication Sign-in Method** to your Facebook app configuration.
+
+In addition, replace the placeholder value for every **FACEBOOK_APP_ID** , **FACEBOOK_CLIENT_TOKEN** and **FACEBOOK_DISPLAY_NAME** keys within your `project.pbxproj` file with their corresponding values.
+
+For additional details, check out the [Firebase authentication](https://firebase.google.com/docs/auth/?authuser=0) page.
 
 ## Twitter
 
@@ -73,4 +83,8 @@ After setting up your [Firebase project](https://flutter.github.io/news_toolkit/
 
 ### Complete the setup
 
-To complete your setup, add the OAuth redirect URI listed in your **Firebase Authentication Sign-in Method** to your Twitter app configuration. For more information, check out the [Firebase authentication](https://firebase.google.com/docs/auth/?authuser=0) page.
+To complete your setup, add the OAuth redirect URI listed in your **Firebase Authentication Sign-in Method** to your Twitter app configuration.
+
+In addition, replace the placeholder values for **TWITTER_API_KEY** and **TWITTER_API_SECRET** inside the `launch.json` file. You must also replace the placeholder value for every **TWITTER_REDIRECT_URI_SCHEME** key within your `project.pbxproj` file with their corresponding values.
+
+For more information, check out the [Firebase authentication](https://firebase.google.com/docs/auth/?authuser=0) page.