expose transport, peer_info and discovery in sc-network (#14132)

* expose transport, peer_info and discovery in sc-network

* fix fmt

* add missing module docs

substrate/client/network/src/transport.rs

@@ -16,6 +16,8 @@
 // You should have received a copy of the GNU General Public License
 // along with this program. If not, see <https://www.gnu.org/licenses/>.
 
+//! Transport that serves as a common ground for all connections.
+
 use either::Either;
 use libp2p::{
 	core::{