Provide guidance on using OAS with Overlay and Arazzo

[handrews]

Many long-requested features for OAS can now be better handled by using the Overlay and/or Arazzo specifications.

I think Arazzo mostly overlaps with the Link Object, but I might be wrong there. Some guidance as to when to use which would probably be good to add.

Overlays solve many requests for global or default values, and if it is reasonably straightforward to do that, we should include info in the OAS pointing to the Overlay spec for that use case. Other use cases include various reference-and-modify constructs (hmm.. do we want to consider pushing this instead of the adjacent-fields-in-Reference-Object approach?)

Putting this in the next patch release, although there may be additional things to do in minor releases here.

[jeremyfiel]

I think Overlay usage is a great target for the many language localization requests over the years.