docs: Some general network install tweaks
First lets add an option for https booting since
its well supported with the libvirt based virtualization
solutions as well as aarch64 UEFI platforms.
Then lets tweak the pxe documentation to refer to the IANA
network boot types, document the aarc64 PXE options, as well
as touching up the grub2 UEFI boot option so it works
with current grub2s.
Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>