reproduce:
        git remote add opencloudos https://gitee.com/OpenCloudOS/OpenCloudOS-Kernel.git
        git fetch --no-tags opencloudos next
        git checkout 4b19dc9296ad0adbeeb0dc91c9bac8feaf384800
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs