reproduce:
        git checkout ffd294d346d185b70e28b1a28abe367bbfe53c04
        b4 shazam https://lore.kernel.org/r/20250126-ppcyaml-v1-9-50649f51c3dd@posteo.net
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs