Add diagram to CellularNonIPSocket.md

Add class hierarchy diagram for consistency.

Author: Amanda Butler

docs/api/networksocket/CellularNonIPSocket.md

@@ -1,5 +1,7 @@
 # Non-IP cellular socket
 
+<span class="images">![](https://os.mbed.com/docs/mbed-os/development/mbed-os-api-doxy/classmbed_1_1_cellular_non_i_p_socket.png)<span>CellularNonIPSocket class hierarchy</span></span>
+
 The CellularNonIPSocket class provides, through standard socket `send` and `recv` member functions, the ability to send and receive 3GPP non-IP datagrams (NIDD) using our cellular IoT feature. This feature is implemented in the [`ControlPlane_netif`](https://os.mbed.com/docs/development/mbed-os-api-doxy/classmbed_1_1_control_plane__netif.html) class.
 
 The constructor takes no parameters. To initialize the socket on a specified NetworkInterface, you must call the `open` method, which takes a CellularContext pointer.