Replace the placeholder "U-Boot instructions" page with initial
documentation about the U-Boot payload. Move the warnings in the
hardware index to this document, but keep a small warning with a link.
This is light on installation instructions compared to what's there on
GRUB, since U-Boot's UEFI support means generic installer images should
work and at that point I think it becomes distro developers' job. But
the added links should be enough as a good start otherwise.
Also add a section for known issues that people might want to know.
Not sure about opening lbmk issues for those, because they're mostly
long-standing upstream things and I'm not really as invested in the
issue tracker.
Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
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