Documentation: Fix x86_64 UEFI outdated references to elilo

Documentation: Fix x86_64 UEFI outdated references to elilo

Problem: The x86_64 UEFI doc references Elilo which is an
unmaintained/orphaned bootloader project. Also, on x86_64 a bootloader
is technically not actually required since there is support for the
Linux EFI stub.

Solution: Remove the references to Elilo from the doc and refer to the
EFI stub doc page, update steps accordingly, and add more details about
creation of the EFI partition to improve clarity.

Signed-off-by: Nir Lichtman <nir@lichtman.org>
Link: https://lore.kernel.org/r/20250108113522.GA897677@lichtman.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>