Documentation: kobject.txt has been moved to core-api/kobject.rst

Moved the `kobject.txt` to `core-api/kobject.rst` and updated the
`core-api` index to point to it.

Signed-off-by: Sameer Rahmani <lxsameer@gnu.org>
[jc: moved it down from the top of core-api/index.rst]
Link: https://lore.kernel.org/r/20200225222125.61874-2-lxsameer@gnu.org


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