Build and host docs for both stable and ToT docs

[jrhemstad]

Today our docs are always ToT. This isn't very delightful.

In an ideal world, our docs would default to show the latest stable release and optionally can select seeing ToT docs.

In an even more ideal world, we'd host multiple versions of our docs, but they are too big right now, so we're going to settle for latest stable + ToT.