reproduce:
        git remote add masahiroy-kbuild https://git.kernel.org/pub/scm/linux/kernel/git/masahiroy/linux-kbuild.git
        git fetch masahiroy-kbuild kbuild
        git checkout masahiroy-kbuild/kbuild
        b4 shazam https://lore.kernel.org/r/20240611175536.3518179-7-masahiroy@kernel.org
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs