LP#2036328: update Antora to 3.1

LP#2036328: update Antora to 3.1

This patch updates the version of Antora used to build the documentation
to a supported release.

* Uses the new @antora/lunr-extension, rather than the antora 2 antora-lunr
* Fixes a few issues that are no longer supported in the new version of
  asciidoctor
* Installs antora via the antora npm package, rather than installing
  @antora/cli and @antora/site-generator separately

Release-note: Update the version of Antora used to build the documentation

Co-authored-by: Jennifer Pringle <jennifer.pringle@bc.libraries.coop>
Co-authored-by: Robin Fitch <rfitch@biblio.org>
Co-authored-by: Andrea Buntz Neiman <abneiman@equinoxOLI.org>
Co-authored-by: Debbie Luchenbill <debbie@mobiusconsortium.org>
Co-authored-by: Lena Hernandez <lena.hernandez@equinoxoli.org>
Signed-off-by: Galen Charlton <gmc@equinoxOLI.org>
Signed-off-by: Jane Sandberg <js7389@princeton.edu>