docs: rewrite resources documentation

Huge overhaul. Includes all new dev guides about how resources are used
on Android and how to provide alternatives, including how to handle
runtime configuration changes (restarts). Plus all new reference
docs for all the resource types (drawables, strings, menus, etc.).

Change-Id: Icf079f5f45b1745a8d54f504e28dbbb52c6f7c96