docs: getting started on android FirebaseUI v10

[russellwheatley]

• Wrote a getting started document for the latest FirebaseUI android auth v10 to mirror closely the previous: https://firebase.google.com/docs/auth/android/firebaseui

[gemini-code-assist]

Summary of Changes

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This pull request introduces a new getting started guide for FirebaseUI Auth v10 on Android. The guide focuses on the new Compose-based authentication library, detailing its integration, configuration, and customization capabilities. It aims to provide developers with a clear path to implement modern authentication flows using Jetpack Compose, while also offering guidance for migrating from previous versions.

Highlights

• New Compose-based Authentication Flow: The recommended sign-in flow for FirebaseUI Auth v10 now leverages Jetpack Compose screens, moving away from Intent builders and ActivityResultLauncher callbacks for a more modern Android development experience.
• Enhanced Features and UI: FirebaseUI Auth v10 introduces Credential Manager integration for faster sign-in, Material 3 UI that can inherit app themes, and built-in flows for multi-factor authentication, account management, linking, and anonymous user upgrades.
• Comprehensive Provider Configuration: The guide details how to configure various authentication providers, including email/password, phone, Google, Facebook, Apple, GitHub, Microsoft, Yahoo, Twitter, anonymous auth, and custom OAuth, with specific setup instructions for Google Sign-In and Facebook Login.
• Customization and Theming: Version 10.x offers significantly more customization options, allowing developers to set themes directly in authUIConfiguration, inherit app themes, or build fully custom AuthUITheme instances.
• Migration Guidance: The documentation provides clear mapping for migrating from older FirebaseUI Auth flows (9.x), explaining how AuthUI.getInstance().createSignInIntentBuilder() and AuthUI.IdpConfig.*Builder() translate to the new Compose-based APIs.

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[gemini-code-assist]

Code Review

The pull request introduces a new GETTING_STARTED.md documentation file for FirebaseUI Auth for Android, detailing its features, setup, provider configuration, sign-in/out flows, customization, and migration guidance. A review comment suggests improving the dependency declarations in this new guide by replacing <latest-version> placeholders with specific, known-working versions to prevent potential build issues and enhance user experience.

[gemini-code-assist]

For a getting started guide, using specific, known-working versions for dependencies is generally more helpful than <latest-version> placeholders. This prevents potential build issues for users who might not know where to find the most recent stable versions. Consider using the specific versions listed in auth/README.md or adding a note on how to find the latest stable versions for these BOMs and libraries.

Suggested change

	implementation(platform("com.google.firebase:firebase-bom:<latest-version>"))
	implementation("com.google.firebase:firebase-auth")
	implementation(platform("androidx.compose:compose-bom:<latest-version>"))
	implementation("androidx.compose.material3:material3")
	// Required only if Facebook Login support is needed
	implementation("com.facebook.android:facebook-login:<latest-version>")
	implementation(platform("com.google.firebase:firebase-bom:32.7.0"))
	implementation("com.google.firebase:firebase-auth")
	implementation(platform("androidx.compose:compose-bom:2024.01.00"))
	implementation("androidx.compose.material3:material3")
	// Required only if Facebook Login support is needed
	implementation("com.facebook.android:facebook-login:16.3.0")