Import augur auto-generated API docs

[jameshadfield]

This is a progress-tracker type issue to track the incorporation of augur auto-generated API documentation which is currently rendered at https://docs.nextstrain.org/projects/augur/en/stable/api/api.html.

I believe we want to use read the docs subprojects for in order to have versioned documentation -- e.g.preserve the API docs for each different augur version & access it via a URL.

[eharkins]

Oops, the commit message there is supposed to say "naive first attempt not using submodules subprojects". I'm hoping my first approach will mesh well with recognizing the augur submodule version using a subproject, i.e. #12 is possible in a way that allows you to control the hierarchy of subproject documents within the parent project.

[eharkins]

a157213 changes the reference/augur heading in the sidebar to a link to https://docs.nextstrain.org/projects/augur/en/migrate-docs/index.html, where all augur reference documents have been consolidated into the augur subproject (and all non-reference documents are not listed / rendered in the subproject or its sidebar since they are instead imported via the git submodule and listed / rendered in the main project).

[eharkins]

This is done for now using subprojects. If #27 pans out, there will be some work again here to change implementation of including these docs so that they show up in this project's table of contents / sidebar instead of having to navigate to a separate url to get to the augur subproject.