Publish documentation

[1ec5]

MapboxSpeech’s public API is used in tandem with MapboxNavigation’s public API, so it should be documented and the documentation should be published somewhere easily accessible. Unlike with MapboxDirections, I think we can rely on the same simple GitHub Pages setup as in mapbox/turf-swift#162. MapboxSpeech is only a minor part of working with MapboxNavigation, and we don’t intend to productize it or the Mapbox Voice API as a standalone product.

/cc @mapbox/navigation-ios @mapbox/docs