Hi @yellowbrickc, welcome to the GitHub Support Community!
It would depend on a lot of factors, like how your documentation is stored, what tools you use to build your sites, etc. Unfortunately Pages doesn’t natively have a built in search plugin so I think you’re going to need to find or build this part, but there are tools available to do so like Google’s Programmable Search Engine:
The simplest solution would be to move any pages/documentation you want to be searchable into the repository that hosts your Pages site. That would mean detaching this documentation from their original repositories though.
You could continue to store the documentation inside the original repositories but you’d likely need some kind of search tool, middleware, or API that you could query when a user performs a search that would search these external repositories. I believe some tools (like PSE) would allow you to do this, but it depends on the tool you’re using/building.