Documentation: add kdump.rst to present crashkernel reservation on arm64

Documentation: add kdump.rst to present crashkernel reservation on arm64

People complained the crashkernel reservation code flow is hard to
follow, so add this document to explain the background, concepts and
implementation of crashkernel reservation on arm64. Hope this can help
people to understand it more easily.

Signed-off-by: Baoquan He <bhe@redhat.com>
Reviewed-by: Zhen Lei <thunder.leizhen@huawei.com>
Link: https://lore.kernel.org/r/20230515060259.830662-3-bhe@redhat.com
[catalin.marinas@arm.com: rewrote some sentences, removed others]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>