resolves #7 set/read xreftext property on page entry in manifest if it differs from title
requested to merge opendevise/oss/antora-atlas-extension:issue-7-xreftext-property-on-page into main
- set xreftext property on page in manifest if value differs from title
- read xreftext property from page in manifest, using title as fallback
- map title property on imported file object to asciidoc.doctitle