[docs] Running mobile app on physical device iOS

Summary: Split the iOS and Android sections apart from each other to reduce the size of the diff. Also, while both sections are related, they are different and should be split.

Test Plan: ran it on physical device

Reviewers: atul, ashoat

Reviewed By: atul, ashoat

Subscribers: ashoat, palys-swm, Adrian, karol-bisztyga, benschac, boristopalov

Differential Revision: https://phabricator.ashoat.com/D2612