reproduce:
        # https://github.com/gourryinverse/linux/commit/71923a8a0d05e8cf00f71c5cb6445c1772ea8f2c
        git remote add gourryinverse https://github.com/gourryinverse/linux
        git fetch --no-tags gourryinverse gourry_cxl_docs
        git checkout 71923a8a0d05e8cf00f71c5cb6445c1772ea8f2c
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs