Link javadocs from the header
Link javadocs from the header

Summary: added a nav button in the header which links to javadocs

Reviewed By: IanChilds

Differential Revision: D4859642

fbshipit-source-id: 9928bb6b2eff9eab2e74b6fcc607cc492d3cf30a

facebook/litho,facebook/litho,facebook/litho,facebook/litho,facebook/litho,facebook/litho