docs(netboot): add full netboot setup guide

Documents the netboot extension: artifact server setup (tftpd-hpa +
nfs-kernel-server), TFTP tree layout, DHCP options 66/67 on the
network DHCP server, userpatches.conf knobs, the
netboot_artifacts_ready hook, a full end-to-end helios64 walkthrough,
and a troubleshooting section.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: iav