Add some API documentation for endusers

[phildarnowsky-broad]

Here we start adding docs covering the basics of using our GraphQL API, including how to fetch all variant frequencies for a gene.

Note the original brief was to replicate the data exported to CSV off the gene page variant table, but I wasn't able to do that exactly, since it turns out some of that data is calculated in the frontend code rather than the API.

[phildarnowsky-broad]

@rileyhgrant this version incorporates all the feedback I got on the demo