reorganized sections, refactored examples into examples/index.md

Thanks to @threeqube for adding all the necessary anchor links for the JSON-LD context. Now I've done a quick rework of the organization of this document, and added a little bit of content here and there. Mainly, moved examples onto the examples page and linked back and forth between them and the definitions they depend on.

See http://ottonomy.github.io/openbadges-specification/ for how it looks if we pull this in.