arm64: add kdump.rst into index.rst

Document kdump.rst was added into Documentation/arm64/, but not listed
in Documentation/arm64/index.rst. That triggers below warning when
executing "make htmldoc":

>> Documentation/arm64/kdump.rst: WARNING: document isn't included in any toctree

Fix it now.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202306110549.ynH2Juok-lkp@intel.com/


Signed-off-by: Baoquan He <bhe@redhat.com>
Fixes: 03dc0e05 ("Documentation: add kdump.rst to present crashkernel reservation on arm64")
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Link: https://lore.kernel.org/r/20230611230358.13635-1-bhe@redhat.com


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>