docs: move the kref doc into the core-api book

This document covers core kernel objects. So, add it into the core-api
book.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/f385af13b4a6d3ff8c89beedd4506900e79ca72e.1588345503.git.mchehab+huawei@kernel.org


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