This introduces v2 docs in order to allow users to opt in to
breaking changes, while still supporting v1 docs for a time
so folks can migrate. At some point v1 doc support will be
removed.
This initial version of v2 docs is experimental. Further
breaking changes will be made before v2 docs are finalized.
A v1-v2 migration guide is included in the documentation.
This also refactors the internal data model to include the full
document structure, such as `metadata` and `schema`, so that
different behavior can be acheived for v1, v2, etc.
Change-Id: Ia0d44ff4276ef4c27f78706ab02c88aa421a307f