docs: enable site search via @antora/lunr-extension

[sgerbino]

Same change as cppalliance/capy#428.

@antora/lunr-extension was already a dependency in doc/package.json but was never registered under antora.extensions in doc/local-playbook.yml, so the built site shipped no search. This registers it, first in the extension list with index_latest_only: true, matching the convention used by Boost.URL, dynamic_bitset, and openmethod.

Verified with a full local build_antora.sh run:

• search-index.js (4.3 MB) is generated at the site root
• _/js/vendor/lunr.js and _/js/search-ui.js are injected by the extension
• Pages render the toolbar search box and load all three scripts

No UI-bundle changes are needed: the Boost UI's templates are gated on SITE_SEARCH_PROVIDER, which the extension sets at build time.

[cppalliance-bot]

An automated preview of the documentation is available at https://371.corosio.prtest3.cppalliance.org/index.html

If more commits are pushed to the pull request, the docs will rebuild at the same URL.

2026-09-25 21:29:39 UTC

[cppalliance-bot]

GCOVR code coverage report https://371.corosio.prtest3.cppalliance.org/gcovr/index.html
LCOV code coverage report https://371.corosio.prtest3.cppalliance.org/genhtml/index.html
Coverage Diff Report https://371.corosio.prtest3.cppalliance.org/diff-report/index.html

Build time: 2026-09-25 21:44:10 UTC