> There’s a nice side-effect, too: The CI definition documents how to compile the project.

Okay, sure, but that's not an argument for CI. That's an argument for a Makefile.

In fact, I find myself going back to Makefiles all the time not in spite of all these specialized build tools, but because of them. Even if the steps are straight-forward for someone who's familiar with the tools, just having a Makefile there in the repository root will be helpful documentation to help someone new (or myself in 2 years) get started with how to build the stuff. For example, for LaTeX, it could be something like:

  build: slides.pdf handout.pdf

  %.pdf: %.tex
    pdflatex $*
    pdflatex $*