Android getting started mirror

The current process for getting Proximity Kit spun up on Android was a bit painful. To ease the pain, add an equivalent Android mirror of the iOS "Getting Started" guide.

Along these lines, it is a bit confusing going to links only to find yourself on iOS only pages. Equivalent Android pages either don't exist or are extremely difficult to find via site navigation. This beings a WIP for including platform switching links on pages that should have both Android and iOS versions.