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 <[e-mail]>
Link: https://lore.kernel.org/r/f385af[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/core-api/index.rst

···
    :maxdepth: 1
 
    kobject
+   kref
    assoc_array
    xarray
    idr

Documentation/core-api/kobject.rst

···
 If all that you want to use a kobject for is to provide a reference counter
 for your structure, please use the struct kref instead; a kobject would be
 overkill.  For more information on how to use struct kref, please see the
-file Documentation/kref.txt in the Linux kernel source tree.
+file Documentation/core-api/kref.rst in the Linux kernel source tree.
 
 
 Creating "simple" kobjects