Add Generator plugin to dump all page data to yaml and json

This is partially based on the sitemap generator. The hope is that it
can help make an intermediate format for many multi-purposed smarter tools
to take advantage of our site data.
(DOC-893) Specify the docs site output format for interop with other tools (v 0.1)
(DOC-894) Specify data input formats for structured documentation (v 0.1)
(DOC-893/894) Improve format version info