docs(dedicated-servers): make the iPXE example chainload netboot.xyz

The custom iPXE script example booted the first local disk in legacy mode
(sanboot --drive 0x80), which merely boots the server's existing disk and is
of little practical use as an example.

Replace it with a script that chainloads netboot.xyz, a network boot menu of
OS installers and live utilities - a far more useful demonstration of what a
custom iPXE script can do:

  #!ipxe
  echo Chainloading netboot.xyz
  dhcp
  chain --autofree https://boot.netboot.xyz