Man pages matching name systemd.nspawn in section 5:

systemd.nspawn.5.en	debian 10 / systemd-container
systemd.nspawn.5.de	debian 11 / manpages-de
systemd.nspawn.5.en	debian 11 / systemd-container
systemd.nspawn.5.de	debian 12 / manpages-de
systemd.nspawn.5.en	debian 12 / systemd-container