Contents of xref should use page title if contents is not specified
If the contents (i.e., text) in an xref macro is not specified, Antora should use the page title as the contents.
While this issue seems simple at first, there's a chicken-egg problem. It's possible that the target page has not yet been loaded, so Antora doesn't know what the page title is. Either Antora as to load all the page titles before starting document conversion, or Antora needs to perform a post-conversion step to fill in page titles where needed.
- If the text of an xref to another page is absent, it is filled in automatically from the title of the target page
- If the text of an xref to a section in another page is absent, it is filled in automatically from the title of the section (potentially with a maximum search depth); if this cannot be achieved in this issue, spawn another issue to deal with it