doc/rados/operations/add-or-rm-mons: simplify the steps to add a mon

doc/rados/operations/add-or-rm-mons: simplify the steps to add a mon

this change removes the step to "ceph mon add" before starting a new
monitor. because the existing leader will start an election at seeing
the MMonJoin message sent by the new joiner, after the quorum is
archieved, the monmap will be updated with the new monitor.
so, "ceph mon add" is not necessary to add a new monitor.
moreover, this command will be blocked until a new quorum is formed,
and the proposed monmap is accepted. but in case of adding a monitor
to a single monitor cluster, the leader will wait until at least two
of the monitors reply to it. apparently, this does not happen unless
the new monitor starts. so from the user's point of view, this
command hangs until timesout, if he/she does not start the mon.b
beforehand. but this is an expected behaviour.

so, to avoid this confusion and simplify the steps to add a new
monitor. we'd better simply remove this "ceph mon add" step.

Fixes: #12620
Signed-off-by: Kefu Chai <kchai@redhat.com>