Doc: Add an example customization for an indeterminate ProgressBar

The indeterminate state for a ProgressBar control was not mentioned
in the customization documentation. Adapt an animation from the old
Qt Quick Controls 1 customization and link to the indeterminate
property documentation.

Fixes: QTBUG-116306
Pick-to: 6.6 6.5
Change-Id: I56d2c7fe4b326637806edd85d40be040b7fdb3f6
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>
(cherry picked from commit 57f5f462fc)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/quickcontrols/doc/snippets/qtquickcontrols-progressbar-custom.qml

@@ -21,11 +21,40 @@ ProgressBar {
         implicitWidth: 200
         implicitHeight: 4
 
+        // Progress indicator for determinate state.
         Rectangle {
             width: control.visualPosition * parent.width
             height: parent.height
             radius: 2
             color: "#17a81a"
+            visible: !control.indeterminate
+        }
+
+        // Scrolling animation for indeterminate state.
+        Item {
+            anchors.fill: parent
+            visible: control.indeterminate
+            clip: true
+
+            Row {
+                spacing: 20
+
+                Repeater {
+                    model: control.width / 40 + 1
+
+                    Rectangle {
+                        color: "#17a81a"
+                        width: 20
+                        height: control.height
+                    }
+                }
+                XAnimator on x {
+                    from: 0
+                    to: -40
+                    loops: Animation.Infinite
+                    running: control.indeterminate
+                }
+            }
         }
     }
 }

src/quickcontrols/doc/src/qtquickcontrols-customize.qdoc

@@ -761,6 +761,9 @@
 
         \snippet qtquickcontrols-progressbar-custom.qml file
 
+        Above, the content item is also animated to represent an
+        \l {ProgressBar::}{indeterminate} progress bar state.
+
 
     \section2 Customizing RadioButton