[OTHER] Consider using Antora Lunr Extension

[ludans]

Instead of using an external search indexing, use https://gitlab.com/antora/antora-lunr-extension

Requires Node.js 16 and Antora 3 #189

Check support for versions of the doc (facets).

Pros:

• indexing is done once, at build time (vs done after publishing the doc site)
• compatible with github pages hosting
• should be more robust

[ludans]

done, will be visible in releases > 4.2.0

todo: validate that it also works on github doc version