doc: mgr: align Ceph-Mgr module vs. plug-in naming
Currently, in Ceph Docs there's a mixture of naming to refer to what originally were "Ceph-Mgr Modules". Apart from the original
module term, in documentation there're plenty references to them as
plugins. As the command-line token for Ceph-Mgr is
module, we might want to keep just that denomination.