docs: Replace dead links to spectre side channel white papers
The papers are published by Intel, AMD and MIPS.
Signed-off-by: Nikola Z. Ivanov <zlatistiv@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250816190028.55573-1-zlatistiv@gmail.com
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