annotations: add references to a set of documents

For preparing annotations on variants in the Ahiqar project, we want to summarize some information that would be redundant to many items in an annotationPage. Therefor we introduced a refs key to list all witnesses on a given annotationPage.

This change has to be reflected in the specs and will require an addition to the

The following discussion from backend!292 (merged) should be addressed:

• @mrodzis started a discussion: (+1 comment)

  This whole structure is not documented.