Move V2 documentation into "/v2" to prep for centralizing doc site

* Both V1 and V2 API documentation will exist as in this repository to
  make compilation and publishing easier.
  * V1 documentation will reside in "/" (for now)
  * V2 documentation will reside in "/v2" (for now)
* Moved all API V2 content into `v2/` subdirectory
* Created a specific compilation rule that uses the new `v2` layout
* Created a barebones layout for any markdown files that haven't been
  moved into v2