docs: zh_TW/index: Move arm64/index to arch-specific section

arm64/index is architecture specific.
Move it to the section "特定體系結構文檔" or "Architecture-specific
documentation".

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Fixes: e5cb9494 ("docs/zh_TW: add translations for zh_TW/arm64")
Cc: Hu Haowen <src.res@email.cn>
Link: https://lore.kernel.org/r/53d86385-b4db-5d02-be6b-795900166f17@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>