You need the DNS and DHCP server module from software center, as it integrates tftp.
It seems the documentation is not updated. The commands should be:
Copy initrd.img and vmlinuz (or the whole images folder content) from ISO to /var/lib/tftpboot/CentOS
Change ownership:
chown -R nobody /var/lib/tftpboot/*
Test pxe server is working by booting a client machine with network boot support:
What remains is to decide where to store the installation files, and how they will be retrieved by the clients (ftp, http, nfs…), tweaking the /var/lib/tftpboot/pxelinux.cfg/default file accordingly.
Note I don’t use it so I might have missed something (uefi…) This was on NethServer 6.8 acting as dhcp server and configuring the ftp server as installation source, just for testing purposes (installing the ftp server module from software center, creating a user, adding the repo boot parameter with the ftp settings to the pxe default file, and copying the installation files over the ftp user path). Hope it helps.