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>

Documentation/core-api/index.rst

@@ -12,6 +12,7 @@ Core utilities
    :maxdepth: 1
 
    kernel-api
+   kobject
    assoc_array
    atomic_ops
    cachetlb