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The current CPU hotplug is outdated.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 <[email protected]>
Cc: Mauro Carvalho Chehab <[email protected]>
Cc: Rusty Russell <[email protected]>
Cc: Srivatsa Vaddagiri <[email protected]>
Cc: Ashok Raj <[email protected]>
Cc: Joel Schopp <[email protected]>
Cc: [email protected]
Signed-off-by: Sebastian Andrzej Siewior <[email protected]>
Signed-off-by: Jonathan Corbet <[email protected]>