Document all extant BoxArt components

[kadamwhite]

API docs are to come, but the documentation site will be considered presentable once it describes the existence and basic concept of each implemented BoxArt and BoxArt Boiler component:

BoxArt Boiler helpers

• Center - Vertically and horizontally center children.
• FullViewport - Help trigger mobile browsers to give the full viewport to games that don't scroll.

BoxArt core components

• Animated
• Stage

Webpack Components

• Sound