Academic papers usually have an introduction just past the abstract and may be the closest to what’s requested here. This is supposed to get the reader prepared for the meat of the paper. In a lot of math papers, it acts as a 101 prepper/refresher for the core concepts used in the paper.
There’s also the wolfram site for math. It’s been a while, but it functions something like an encyclopedia of all math topics. I’ve used it to quickly backfill a reference many, many times.
Reading an article or proof is not exactly analogous to “reading the source” as both articles and proofs are meant to be written to be read by a human with an agreed upon background and lead them logically to the conclusion(s) of the article. Articles and even proofs can be intimidating, I get that, but they are surmountable with a suitable time investment.