Followup to issue #3077985: better annotation structure, extra examples, reorganization and renames

This is a grab bag of a pretty large refactoring.

I took many of @Wim Leers's suggestion from #3077985: The \Drupal\jsonapi_hypermedia\Annotation\JsonapiHypermediaLinkProvider annotation and I also resolved the nits in #3079225: Derivers nits. Unfortunately, it would be pretty inefficient at this point for me it's very difficult to make smaller commits that directly address each point.

The reason is that I moved the two default plugin implementations into an example directory and so it's hard to get a diff that has the internal file changes.

Taking the lead from @e0ipso in contrib JSON:API, I'm gonna make a few extra, empty commits under Wim's name to reflect his big contribution to these changes.