doc/cephadm: document the cephfs-proxy sidecar

The Tentacle notes announce an automatically deployed cephfs-proxy
daemon, but the only documentation was the developer design page.
Add an operator section to the SMB service docs covering why the
proxy exists, when it is deployed, and what to look for when
troubleshooting, and link it from the smb module provider field and
the release notes.

Fixes: https://tracker.ceph.com/issues/79041

Signed-off-by: Emmanuel Ameh <eameh@contractor.linuxfoundation.org>