reproduce:
        git remote add opencloudos https://gitee.com/OpenCloudOS/OpenCloudOS-Kernel.git
        git fetch --no-tags opencloudos next
        git checkout cf5d03932d492065885a8daef93b8f4b3f26176c
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs