Decouple this Repo from cohortextractor

[ghickman]

We currently install cohortextractor and it's dependencies so we can use automated codegen tooling for pages such as the variable reference. This means the dev tooling is tied to cohortextractor's requirements, which are not entirely simple. It also couples cohortextractor to any requirements from this repo.

Databuilder avoided this coupling by exporting a JSON document with all necesssary docs data and having a plugin render it to markdown.

Now that we've proven the technique is workable with databuilder we can move cohortextractor to it as well.