Searching within a site requires a server-side program to index the site and another to search the index database and spit out the results. It's not trivial. You can get pre-made search programs (like Zoom from Wrensoft) or use a service. If you host your own scripts, you'll have more flexibility with the output (including no ads), but you have to set it all up. Search services are either paid or show ads. I decided to use Google Custom Search because it's free and fairly easy. I may switch later since it doesn't look like I can set up regular indexing the way I could with atomz. :-(
It was a bit confusing setting up the custom search within the Bootstrap nav bar -- it took an hour or two to figure out how to set up the search box on one page and the search results on another,and the formatting looked odd until I found info on the Bootstrap site about box formatting.
more info:
