add iOS documentation

Fix #132 (closed)

I've decided to not use cargo-lipo as it describe itself as deprecated. Instead it requires editing the project config file to add a few elements.

I don't own any iOS device so it was tested only using an emulator. If someone could test it on real hardware (and on simulator on M1), it would be amazing (using this example project and checking out the branch ios is enough if you are in a hurry). I'll rename arti-android-example to arti-mobile-example once this get merged so both platform share the same repository. Feel free to copy this project in a more official place if you want.

There is a section about forwarding logs to OSLog. I wasn't actually able to verify it's working properly. For some reason I did not manage to get logs from the Simulator, be it from Arti or from direct calls to logging from Swift