Documentation: move timer related documentation to a single place

We have two directories with timer related information in
Documentation/: hrtimers/ and hrtimer/. timer_stats are not restricted
to hrtimers. Move all those files into Documentation/timers where we
can pile up other timer related docs as well.

Pointed-out-by: Randy Dunlap <randy@oracle.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>