LP2056480: docs: only index the latest version of the docs for lunr

LP2056480: docs: only index the latest version of the docs for lunr

This addresses a regression caused by bug 2056480.

To test:
* Build the docs using
  cd docs
  perl generate_docs.pl --base-url=http://example.com
* Open docs/output/index.html in your browser
* Type a search into the search box
* Note that you only get results for latest, rather than getting duplicate
results from multiple versions.
* Note that the file docs/output/search-index.js is now 3.3 MB, rather than
5.53 MB.

Signed-off-by: Jane Sandberg <sandbergja@gmail.com>
Signed-off-by: Galen Charlton <gmc@equinoxOLI.org>