Docs/translations/ko_KR: link memory-barriers wrapper with other language versions

The menu for documents of other available languages is created for
documents in same file hierarchy under the translations/ directory.
Because memory-barriers.txt of Korean translation is at the root index
of translations/ directory while that for English is on
core-api/wrappers/memory-barriers.rst, the Korean translation is not
linked with other available language versions via the menu.  Move the
document under the same directory hierarchy to make it linked with other
language versions.

Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240826010949.78305-2-sj@kernel.org