This augments the JSONHTMLWriter found in

sphinxcontrib-serializinghtml (or native to Sphinx in the

old-but-still-supported 1.8) to include two new pieces of information:


A structured representation of the Table of Contents for the whole

   doc bundle.


Rendered HTML for the anchors in the page (similar to the "toc"

   available in the page, but without the page's own header).


These can be used by consumers (including our website) to pull out this

structured information without having to parse the docsets themselves.

Unit tests pass on all supported versions of Sphinx and Python.
Built the docs and checked for build errors, bad links, and presentation

issues.