Documentation: Update CPU hotplug and move it to core-api

Documentation: Update CPU hotplug and move it to core-api

The current CPU hotplug is outdated. During the update to what we
currently have I rewrote it partly and moved to sphinx format.

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Srivatsa Vaddagiri <vatsa@in.ibm.com>
Cc: Ashok Raj <ashok.raj@intel.com>
Cc: Joel Schopp <jschopp@austin.ibm.com>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>