Rewrite front page to include a better description and basic examples

[awwright]

I'd like to come up with a few different ideas for ways to rework the front page so people who don't know what JSON Schema is both (1) understand where they would use it and (2) are introduced to the core concepts (like the recursive nature, $ref, usage with HTTP, and the default-validates/empty-schema concept) as early as possible.

[awwright]

This has sort of been implemented in the major update recently