As part of our migration of all nextstrain-docs to RTD and docs.nextstrain.org
this commit switches Auspice to using RTD.
There is further work needed in terms of organisation structure, using pages here
under docs.nextstrain.org's RTD page (as opposed to the Auspice subproject), but this
migration paves the way.
No content changes, but lots of syntax changes etc
See https://github.com/nextstrain/docs.nextstrain.org/issues/8 for context.