doc: move split_brain.md to debugging dir.

Change-Id: I4558514bca9365b4563e74f166f2a0156f798d0f
Signed-off-by: hchiramm <hchiramm@redhat.com>
Reviewed-on: http://review.gluster.org/8544
Reviewed-by: Prashanth Pai <ppai@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>