As the number of posts is growing, I think that having a search engine within the website is now relevant. This post only shows what I am using for that.
Instead of using an external search engine such as Google or DuckDuckGo, you have a plugin in Nikola than enables searching on the client side by using jQuery.
Assuming you have installed Nikola > v7.0, you can install the plugin like this:
It will give you a configuration sample to paste in your config.py file.