Add network socket images

Add class hierarchy images to network socket user APIs.

Author: AnotherButler

docs/reference/api/connectivity/networksocket/CellularInterface.md

@@ -1,5 +1,7 @@
 <h2 id="cellular-api">Cellular</h2>
 
+<span class="images">![](https://os.mbed.com/docs/v5.8/mbed-os-api-doxy/class_cellular_base.png)<span>CellularBase class hierarchy</span></span>
+
 The CellularBase provides a C++ API for connecting to the internet over a Cellular device.
 
 Arm Mbed OS provides a [reference implementation of CellularBase](https://os.mbed.com/docs/v5.8/mbed-os-api-doxy/_easy_cellular_connection_8h_source.html), which has more information.

docs/reference/api/connectivity/networksocket/EthInterface.md

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 ## Ethernet
 
+<span class="images">![](https://os.mbed.com/docs/v5.8/mbed-os-api-doxy/class_eth_interface.png)<span>Ethernet class hierarchy</span></span>
+
 The EthInterface provides a C++ API for connecting to the internet over Ethernet.
 
 ### EthInterface class reference

docs/reference/api/connectivity/networksocket/MeshInterface.md

@@ -1,5 +1,7 @@
 <h2 id="mesh-api">Mesh</h2>
 
+<span class="images">![](https://os.mbed.com/docs/v5.8/mbed-os-api-doxy/class_mesh_interface.png)<span>MeshInterface class hierarchy</span></span>
+
 The Arm Mbed mesh API allows the application to use the IPv6 mesh network topologies through the [nanostack](/docs/v5.8/reference/technology.html#nanostack) networking stack.
 
 **Tips:**

docs/reference/api/connectivity/networksocket/TCPServer.md

@@ -1,5 +1,7 @@
 ## TCPServer
 
+<span class="images">![](https://os.mbed.com/docs/v5.8/mbed-os-api-doxy/class_t_c_p_server.png)<span>TCPServer class hierarchy</span></span>
+
 The TCPServer class provides the ability to accept incoming TCP connections. The `listen` member function sets up the server to listen for incoming connections, and the `accept` member function sets up a stateful TCPSocket instance on an incoming connection.
 
 The constructor takes in the NetworkStack pointer to open the socket on the specified NetworkInterface. If you do not pass the NetworkStack pointer in the constructor, then you must call open to initialize the socket.

docs/reference/api/connectivity/networksocket/TCPSocket.md

@@ -1,5 +1,7 @@
 ## TCPSocket
 
+<span class="images">![](https://os.mbed.com/docs/v5.8/mbed-os-api-doxy/class_t_c_p_socket.png)<span>TCPSocket class hierarchy</span></span>
+
 The TCPSocket class provides the ability to send a stream of data over TCP. TCPSockets maintain a stateful connection that starts with the `connect` member function. After successfully connecting to a server, you can use the `send` and `recv` member functions to send and receive data (similar to writing or reading from a file).
 
 The constructor takes in the NetworkStack pointer to open the socket on the specified NetworkInterface. If you do not pass in the constructor, then you must call `open` to initialize the socket.

docs/reference/api/connectivity/networksocket/UDPSocket.md

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 ## UDPSocket
 
+<span class="images">![](https://os.mbed.com/docs/v5.8/mbed-os-api-doxy/class_u_d_p_socket.png)<span>UDPSocket class hierarchy</span></span>
+
 The UDPSocket class provides the ability to send packets of data over UDP, using the `sendto` and `recvfrom` member functions. Packets can be lost or arrive out of order, so we suggest using a [TCPSocket](/docs/v5.8/reference/tcpsocket.html) when you require guaranteed delivery.
 
 The constructor takes in the NetworkStack pointer to open the socket on the specified NetworkInterface. If you do not pass in the constructor, then you must call `open` to initialize the socket.

docs/reference/api/connectivity/networksocket/WifiInterface.md

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 ## Wi-Fi
 
+<span class="images">![](https://os.mbed.com/docs/v5.8/mbed-os-api-doxy/class_wi_fi_interface.png)<span>WiFiInterface class hierarchy</span></span>
+
 The WifiInterface provides a simple C++ API for connecting to the internet over a Wi-Fi device.
 
 There are multiple [Wi-Fi components](https://os.mbed.com/components/cat/wifi/) that implement the WiFiInterface class. The example below uses the [ESP8266Interface](https://github.com/armmbed/esp8266-driver).