docs: enable site search #401

From the review result:

Try to provide search even if it just run google search on the domain like cppreference.

Source: Boost review result (conditional acceptance), Boost mailing list, September 2026.

@antora/lunr-extension is already a dependency in doc/package.json but is not registered under antora.extensions in doc/local-playbook.yml, so the built site ships no search. Likely a small playbook change plus UI verification.