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.

[nginx-jack]

We're currently using Coveo on docs.nginx.com. Happy to look into integration for the unit docs.

[Jcahilltorre]

The search at the top of docs.nginx.com already returns results for Unit docs. The missing part of the implementation is to add the Search field to the Unit docs sphinx theme