- squash-merge the improve_toc branch, which moves all the Sphinx styling

- squash-merge the improve_toc branch, which moves all the Sphinx styling
and extensions into an external library, and also reorganizes most large documentation
pages into many small areas to reduce scrolling and better present the context
into a more fine-grained hierarchy.

(cherry picked from commit 9561321d0328df270c4ff0360dc7a035db627949)

- manually port previous differences between 0.9 and 1.0 docs to the
cherry-pick.