docs: Start documenting API using OpenAPI

docs: Start documenting API using OpenAPI

When the REST API was first added, we attempted to document it using
OpenAPI 2.0 (formerly Swagger). This was mostly never completed because
(a) it was really tedious and (b) no one was that bothered. However, as
we expand the range of clients for the REST API, having a well
documented API becomes more and more of an asset.

Start doing this by adding a brand new schema, this time using OpenAPI.
This will entirely replace the older schema and, as such, is namespaced
separately. We start by documenting '/' (i.e. the index) page and will
add additional resources as we go.

Signed-off-by: Stephen Finucane <stephen@that.guru>