Dedup iOS/Android docs

Some docs in the arti repo are duplicated. They seem like they've diverged a bit, and it would probably be best to deduplicate them before they diverge further. Two examples that I've noticed are:

• doc/Android.md / web/docs/integrating-arti/custom-wrappers/android.md
• doc/iOS.md / web/docs/integrating-arti/custom-wrappers/iOS.md

I'm not sure the best way to dedup these, since the web docs need some front matter whereas the dev docs don't.