LUDOC-550 sec: doc update for nodemap mapping offset

LUDOC-550 sec: doc update for nodemap mapping offset

This patch adds the "Defining Mapping Offsets" section in the Lustre
Operations Manual to introduce mapping offset capability as
implemented by LU-18109.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I7ff2aeb5f32b2783ae27a54a7ff3f1ed4b4f9335
Reviewed-on: https://review.whamcloud.com/c/doc/manual/+/58483
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Marc Vef <mvef@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>