samples: move timers example code from Documentation

Move timers examples to samples and remove it from Documentation
Makefile. Create a new Makefile to build timers. It can be built
from top level directory or from timers directory:

Run make -C samples/timers or cd samples/timers; make

Acked-by: Jonathan Corbet <corbet@lwn.net>
Acked-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>