Doc: dead link in iPXE Doc on main branch

[nicop311]

Hello all :smiley: There is a dead link on main branch in the file docs/IPXE.md:

At this line, link [ipxe/ipxe/](../ipxe/ipxe/) is dead. And I don't know where the .h files are or if they exist on main branch.

However on branch/tag v0.6.0, the folder ipxe/ipxe/ exist with the *.h files.

[jacobweinstock]

Hey @nicop311, thanks for reporting this. We've actually moved ipxe building to another repo. https://github.com/tinkerbell/ipxedust We need to clean up this bad link. The doc lives here: https://github.com/tinkerbell/ipxedust/blob/main/docs/IPXE.md