doc/rados: add "change public network" procedure

doc/rados: add "change public network" procedure

Add a procedure to /doc/rados/operations/add-or-rm-mons.rst that
explains how to change the public_network in a Ceph cluster deployed
with cephadm. This procedure was developed by Eugen Block, and can be
seen in its original form here:
https://heiterbiswolkig.blogs.nde.ag/2024/02/22/cephadm-change-public-network/

Co-authored-by: Eugen Block <eblock@nde.ag>
Signed-off-by: Zac Dover <zac.dover@proton.me>
(cherry picked from commit 0b6739a4aa2dd931d97b8af9b3a352969f8767c2)