doc: move the cephfs-proxy section to the smb module page

Per review, the mgr/smb page is the better home since the smb module
drives the feature flag; the cephadm service page keeps a short
pointer. Also drop the sub-daemon and regular-image phrasing and
state that the sidecar is always colocated with its Samba instance.

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