I did lunr.js search for a static documentation website (Hugo) a month ago. It is easy to implement and works well, except one thing - it does not generate matching text fragment out of the box.
There is a workaround - to index sections instead of whole pages, but it requires parsing documentation pages, and may create constraints for document structure.
@fxn @Niklas what about a POC with elasticsearch? Stay tunned on this issue: [POC] Search in rails guides · Issue #39252 · rails/rails · GitHub
And obviously feel free to ask any related change to see if can be done 
I created a PR of a client side search using lunr.js which works pretty well: Add search to Rails Guides #39299, including a Demo.
Am I missing something or this is still not available? Maybe RoR team is waiting for the release of Active Record Search to enable this 