doc: add missing files to timers/00-INDEX

- timers-howto was added by commit 0fcb8081 (Documentation: Add
  timers/timers-howto.txt)
- NO_HZ was added by commit 0c87f9b5

 (nohz_full: Add documentation.)

Cc: Patrick Pannuto <ppannuto@codeaurora.org>
Signed-off-by: Henrik Austad <haustad@cisco.com>
Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>