I would be good if you enable your users, making technical inquiry; to also see whether the above resources have any coverage of that specific topic (perhaps in an adjacent window). This is above and beyond your own resources.
Our goal indeed is to build and expand a community for all needs around data science and machine learning experts. A universal search that encompasses more resources would definitely help this goal. Unfortunately, we do not own these other properties, so we cannot add the necessary markup to create a meaningful search indexing.
Some search engines offer users to create custom search engines that limit their search scope to certain domains, e.g. Google's https://developers.google.com/custom-search . Maybe that's something for you to look at.