reproduce:
        # https://github.com/intel/linux-intel-lts/commit/c36cad6eceb9b8b5ac81ccf043b06f0772c41182
        git remote add intel-lts https://github.com/intel/linux-intel-lts.git
        git fetch --no-tags intel-lts 6.1/dovetail-xenomai
        git checkout c36cad6eceb9b8b5ac81ccf043b06f0772c41182
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs