We have been working on a free and open source tool for writing build instructions for hardware projects called GitBuilding.
This was borne out of documenting our OSHW 3D-printable microscope and precision positioning system. It’s essentially a static site generator, so it’s a good fit if you want to host documentation on GitHub or GitLab pages. It does provide a PDF output as well (though this is still somewhat experimental).
It’s early days, so to try it out you need to be comfortable with command-line tools and using Python’s pip installer. We are also working on the structure and syntax and may still change things around. We are however close to migrating our own documentation to it, so will be as annoyed as you if we need to make breaking changes.
At this stage we’d love for people to try it out, gather feedback and better understand other people’s use-cases for something like this.