Read The Docs can't build our docs

[mythmon]

As of #1820, Read The Docs has started failing to build our docs, because it lacks the C library that the new faster crypto library is using.

Here's an example of a failed build: https://readthedocs.org/projects/normandy/builds/8850032/

I'm not quite sure what the way forward here is. It looks to me like we might be able to configure the Docker image that RTD uses to build our docs. If we could do that, we could include whatever libraries we want.

Another option would be to try and carefully split our docs further so that we don't have to install that package in the doc builder.

If those don't work, maybe we'll have to look into building the docs ourselves, or some other solution so that we control the environment more closely.

[mythmon]

We've moved the docs to https://mozilla.github.io/normandy/