kho: docs: combine concepts and FDT documentation

kho: docs: combine concepts and FDT documentation

Currently index.rst in KHO documentation looks empty and sad as it only
contains links to "Kexec Handover Concepts" and "KHO FDT" chapters.

Inline contents of these chapters into index.rst to provide a single
coherent chapter describing KHO.

While on it, drop parts of the KHO FDT description that will be superseded
by addition of KHO ABI documentation.

[rppt@kernel.org: fix Documentation/core-api/kho/index.rst]
Link: https://lkml.kernel.org/r/aV4bnHlBXGpT_FMc@kernel.org
Link: https://lkml.kernel.org/r/20260105165839.285270-4-rppt@kernel.org
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Pratyush Yadav <pratyush@kernel.org>
Cc: Alexander Graf <graf@amazon.com>
Cc: Jason Miu <jasonmiu@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Pratyush Yadav <pratyush@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>