OAS 3.2: $self keyword

Do we want to talk about the new `$self` keyword, which provides the self-assigned URI of the OpenAPI document, which also serves as its base URI for `$ref` resolution.

There are some product considerations also, but we've not really got stuck into this sort of thing in the OpenAPI documentation yet and its probably way off in the deepend compared to other things we could talk about. E.g we only just added pagination in #359.