Replace the now-unavailable Documentation CI step

[mattpolzin]

The Documentation CI step used to take code-documentation and produce a Wiki with pages documenting all the parts of OpenAPIKit. This was based on the swift-doc project which has been discontinued. I'd like to generate some nice documentation still and I'm open to whatever so if you have an idea you'd like to implement let's discuss here briefly just to make sure I understand your plans!

[mattpolzin]

This will be set up from v4.0.0 of OpenAPIKit and onward. That was just the easiest option since v3.x of OpenAPIKit still supports back to Swift version 5.1 and uses an old version of the SwiftPM framework as well.