doc/cephadm: improve "Activate Existing OSDs"

doc/cephadm: improve "Activate Existing OSDs"

Improve the section "Activate Existing OSDs".

Supplement the information in the "Activate Existing OSDs" section with
a procedure developed by Eugen Block, here:
https://heiterbiswolkig.blogs.nde.ag/2025/02/06/cephadm-activate-existing-osds/

This procedure explains how to activate OSDs on a host that, for
whatever reason, has had to have its operating system reinstalled.

Co-authored by Eugen Block.

Signed-off-by: Zac Dover <zac.dover@proton.me>