Man pages are missing from PyPI source tarball

[Conan-Kudo]

Trying to build the latest release in Fedora leads to this error:

+ rm -rfv /builddir/build/BUILDROOT/kiwi-boxed-plugin-0.2.12-1.fc35.noarch/usr/bin/__pycache__
+ make buildroot=/builddir/build/BUILDROOT/kiwi-boxed-plugin-0.2.12-1.fc35.noarch/ docdir=/usr/share/doc/ install
# install plugin manual page and license/readme
# NOTE: this file is not handled through pip because on system level
install -d -m 755 /builddir/build/BUILDROOT/kiwi-boxed-plugin-0.2.12-1.fc35.noarch/usr/share/man/man8
gzip -f doc/build/man/kiwi::system::boxbuild.8
gzip: doc/build/man/kiwi::system::boxbuild.8: No such file or directory
make: *** [Makefile:19: install] Error 1

The tarball doesn't include the files, so it looks like we're not generating them before making the sdist tarball to upload.