Move iOS docs to top, add Android placeholders (#8511)

* Temporarily remove using-executorch-ios.md

* Move Apple runtime docs to new location

Author: GregoryComer

docs/source/apple-runtime.md

@@ -1,3 +1,23 @@
 # Using ExecuTorch on Android
 
-Placeholder for top-level Android documentation
+To use from Android, ExecuTorch provides Java API bindings and Android platform integration, available as a AAR file. The ExecuTorch C++ APIs can also be used from Android native.
+
+## Installation
+
+TODO Instructions on downloading the pre-built AAR. Replace with Maven/Gradle package management when available.
+
+### Building from Source
+
+TODO Instructions on re-creating and customizing the Android AAR.
+
+## Android Backends
+
+TODO Describe commonly used backends, including XNN, Vulkan, and NPUs.
+
+## Runtime Integration
+
+TODO Code sample in Java
+
+## Next Steps
+
+TODO Link to Java API reference and other relevant material