Add a search engine to the docs

[fredt34]

I love nginx-unit and the docs are good and clear; a search engine (such as https://docs.nginx.com/nginx/admin-guide/) would be a huge improvement and allow more users.

[Pavlusha311245]

I will support the initiative. I was just leafing through the document and thought about this.
I would probably like to see something like Algolia.