Rename MQTT Subscriptions Example and update screenshot

According to the guideline "Example" should be not part of the name of
the example - remove it.

Add example to the Connectivity category.

Update the screenshot as the current one is obsolete. The application
no longer has a ping button but it has username and password fields.

Task-number: QTBUG-110893
Change-Id: Id9608ea6e76a3a7e89b0fd491d9fb329cfcbedc3
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>

examples/mqtt/doc/simpleclient.qdoc

@@ -47,8 +47,8 @@
     \printuntil }
 
     In this example, we subscribe to all topics. For more information about
-    how to receive messages on particular topics, see
-    \l {MQTT Subscriptions Example}.
+    how to receive messages on particular topics, see the
+    \l {MQTT Subscriptions} example.
 
     For an example of how to use the QMqttClient class in a Qt Quick
     application, see \l {Quick MQTT Example}.

examples/mqtt/doc/subscriptions.qdoc

@@ -3,7 +3,8 @@
 
 /*!
     \example subscriptions
-    \title MQTT Subscriptions Example
+    \examplecategory {Connectivity}
+    \title MQTT Subscriptions
     \ingroup qtmqtt-examples
     \brief Creating a application that communicates with an MQTT broker.