Support link= in images macro with xref notation
I often wish to use a link to a resource from a click on an image. Sometimes I wish to refer to the image itself (to allow it to be expanded or shown on its own tab in the browser).
Typically in Asciidoctor I would use expressions like the following:
link= needs a general content referencing schema/resource id for internal resources in Antora which could be an image or it could be a pdf attachment or (for example) a video or something else. It could equally also be a page (so extending the xref idea from page ids to being generic resource ids makes a lot of sense).
This is somewhat similar and provides an argument for a generic xref scheme as discussed in #227 (comment 108558788)