reproduce:
        git checkout 2014c95afecee3e76ca4a56956a936e23283f05b
        b4 shazam https://lore.kernel.org/r/20250207-ppcyaml-v2-6-8137b0c42526@posteo.net
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs