Unfortunately you are never going to see a project with comprehensive documentation, I'm not sure what that would even look like.

It probably would look like Tex, FWIW.

Edit: I move to the bookcase to look at TeX: The Program. There it is sitting next to my volumes of TAOCP. A reminder of my failures. I can almost feel the disaproving gaze of D. E. Knuth. I'm not worthy.