Move github hosted markdown documents to docs.nextstrain.org

[eharkins]

Various repositories [1] have markdown documentation, README content and other similar documents, which can only be viewed here on github. In many cases we will want to keep content in these documents on github only since it really only pertains to interfacing with that repository on github. In some other cases, it might behoove us to make some of this content more accessible by hosting it on docs.nextstrain.org and linking to it from README files.

[1] - List of such documents (may need updating): https://docs.google.com/document/d/1hq6hjukg3Pw8m12Y0IaephQYJCFvpFrChl0K0B_4UrQ/edit#heading=h.68a4ghmtefnx