Add minimal Jetpack Compose quickstart for the Android SDK

[fungc-io]

Adds a new page Android SDK with Jetpack Compose (get-started/native-mobile-app/android/jetpack-compose.md) as a sub-page under Android SDK, plus its SUMMARY.md entry.

Why

The main Android SDK guide is View/XML + callback listeners, but the linked example app is now Jetpack Compose. This page gives Compose developers a walkthrough that matches the modern UI toolkit without forcing them to translate XML to Compose.

Design: deliberately minimal

It uses the SDK's coroutine suspend API from a single @Composable with rememberCoroutineScope() + LaunchedEffect and plain remember { mutableStateOf(...) } — no ViewModel/StateFlow/DI. That keeps the Authgear integration front and center and the page short. It links the example repo for readers who want the fuller ViewModel-based structure.

Notes

• Reuses the main guide's setup (portal app, Maven Central dependency, redirect URI) via links rather than duplicating them.
• Includes the AppCompat window-theme step up front — Compose apps commonly hit the OAuthActivity "You need to use a Theme.AppCompat theme" crash without it.
• Uses typed SessionState.AUTHENTICATED and the Maven Central coordinate com.authgear:android-sdk:3.0.0.

Opening for review (not merged) — it's a new page and a placement/editorial call.