docs(TwoFactorAuth): Add 2FA doc for integration in a new app

TwoFactorAuth/README.md

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+# 2 factor authentication
+
+## How to integrate into a new app (Part One, polling-only)
+
+This enables the feature, that tries to retrieve any ongoing login challenge when the app is brought to the foreground.
+It does NOT include notifications support, which is an extra (Part Two).
+
+### 1. Add the dependencies
+
+Depend on these 2FA related libraries:
+
+```kotlin
+implementation(project(":Core:TwoFactorAuth:Front"))
+implementation(project(":Core:TwoFactorAuth:Back:WithUserDb"))
+```
+
+### 2. Define the TwoFactorAuthManager singleton
+
+#### A. With the user db dependency
+
+Example:
+
+```kotlin
+/**
+ * Singleton for incoming 2FA (two factor authentication) challenges.
+ *
+ * Not a ViewModel because the state needs to be scoped for the entire app.
+ */
+val twoFactorAuthManager = TwoFactorAuthManager { userId -> AccountUtils.getHttpClient(userId) }
+```
+
+#### B. With NO dependency on the user db
+
+If you can't or don't want to depend on the user database dependency, you need to provide several more parameters.
+Here's an example.
+
+```kotlin
+val twoFactorAuthManager = TwoFactorAuthManager(
+    coroutineScope = coroutineScope,
+    userIds = someStorage.connectedUsers.map(mutableSetOf()) { users ->
+        users.map { it.id }
+    }.distinctUntilChanged(),
+    getAccountInfo = {
+        val info = getUserAccountInfoById(it)
+        ConnectionAttemptInfo.TargetAccount(
+            avatarUrl = info.avatar,
+            fullName = info.displayName,
+            initials = info.computeInitials(),
+            email = info.email,
+            id = it.toLong(),
+        )
+    },
+    getConnectedHttpClient = { userId -> getHttpClientForUser(userId) }
+)
+```
+
+### 3. Add the overlay where needed
+
+For each Activity in the app (including login screen for multi-account apps):
+
+Add `TwoFactorAuthApprovalAutoManagedBottomSheet(twoFactorAuthManager)` at the root of the content.
+
+Example:
+
+```kotlin
+class MainActivity : ComponentActivity() {
+    override fun onCreate(savedInstanceState: Bundle?) {
+        super.onCreate(savedInstanceState)
+        setContent {
+            TwoFactorAuthApprovalAutoManagedBottomSheet(twoFactorAuthManager) // References the singleton declare just before.
+            Whatever()
+        }
+    }
+}
+```
+
+If the Activity is NOT using Compose, or if you don't know (e.g. in a `BaseActivity` class), the `addComposableOverlay`
+function was made just for that, just make sure it's called after `setContentView` or `setContent`.
+
+Example:
+
+```kotlin
+class MainActivity : ComponentActivity() {
+    override fun onCreate(savedInstanceState: Bundle?) {
+        super.onCreate(savedInstanceState)
+        setContentView(whatever)
+        addComposableOverlay { TwoFactorAuthApprovalAutoManagedBottomSheet(twoFactorAuthManager) }
+    }
+}
+```
+
+At this point, you can expect the feature to be working (albeit without notifications).
+
+## How to integrate into a new app (Part Two, notifications support)
+
+**⚠️⚠️ NOTE: ⚠️⚠️** 
+
+Unless specified otherwise, the **code below is to be put into Google Play Services dedicated source sets**.
+
+For example, if the host app is published to F-Droid + the Google Play Store and has 2 product flavors named "fdroid" and "standard", where "standard" contains Google Play Services dependent code, by default, the code below should be located into `app-module/src/standard/kotlin`, where `app-module` is the target app or library module.
+
+### 1. Add the right dependency in the right configuration
+
+Add this dependency in the host app's Gradle build file:
+
+```kotlin
+"standardImplementation"(project(":Core:Notifications:Registration"))
+```
+
+Here, "standard" matches the product flavor that includes Firebase/Google Play Services dependencies.
+
+### 2. Ensure notification token and topics will be synced
+
+First, create the `RegisterUserDeviceWorker` class (if it doesn't exist already). It needs to subclass `AbstractNotificationsRegistrationWorker`.
+
+You can find the Mail app example here: https://github.com/Infomaniak/android-kMail/blob/e272d5b5ff5f4b1eb1f11784537a3d624c063e80/app/src/standard/java/com/infomaniak/mail/firebase/RegisterUserDeviceWorker.kt
+
+Follow other changes from this PR in the Mail, or the kDrive app:
+https://github.com/Infomaniak/android-kDrive/pull/1872/changes
+https://github.com/Infomaniak/android-kMail/pull/2698/changes
+
+Here's the list of changes you need to add from the example PRs above:
+- Ensure `TwoFactorAuthNotifications.channel()` is created/submitted to Android's NotificationManager.
+- Ensure `NotificationsRegistrationManager` is added in `userDataCleanableList` at the beginning of the app process for the Play Services app variant.
+- Ensure the user addition and removal functions to call `resetForUser` in elements registered `userDataCleanableList` (already done if added Cross-app login first).
+- Ensure `NotificationsRegistrationManager.scheduleWorkerOnUpdate` is called in an app process wide coroutine, right from the app process start.
+- Ensure the `Application` subclass is declared properly in the manifest for the Play Services dependent product flavor
+- In the `FirebaseMessagingService` subclass (create it and declare it in the manifest if needed), make sure the `onNewToken` and `onMessageReceived` functions are implemented properly to forward the new tokens to `NotificationsRegistrationManager`, and matching notifications (the ones with `TwoFactorAuthNotifications.TYPE` in key `"type"`), forwarded to `twoFactorAuthManager.onApprovalChallengePushed(…)`.
+- Ensure any other push notification topics are properly integrated for `RegisterUserDeviceWorker` and `NotificationsRegistrationManager.scheduleWorkerOnUpdate`.