This allows backwards compatible changes in the future.

The `X-` prefix is deprecated as of https://datatracker.ietf.org/doc/html/rfc6648

Currently callback expressions don't allow referencing other operations

So we have to include it separately. It's a little more text.

See https://github.com/OAI/OpenAPI-Specification/issues/417

Closes #81

* The "old" Destination is now in destination-any but still called Destination.
* There is now a PublicDestination (w/o contactInfo and callback) and a PrivateDestination
* Removed notice about missing fields depending on scope (since its now obvious from the schema)

Reasoning: It't not version 20 but version 2.

Introduces multiple regions per service.

https://git.fitko.de/fit-connect/api/-/issues/86

The `externalDocs` element allows referencing an external
resource for extended documentation. Since we do it, lets
refer to it.