Cross Site Search?

I have been searching for a plugin or other way to implement cross site search on my wordpress site. Specifically, I have a support site that is not on WordPress that contains docs that I would like to appear on my WordPress site search. Is this even possible? I’ve seen several plugins for mulitsite searches, but they all require the sites to be on WordPress. I’m not even sure I am searching for the correct type of plugin. Any help or guidance is much appreciated!