OAS 3.2: $self keyword

[philsturgeon]

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.