Improve UIInputModule documentation and examples

Only the examples that are reused across the class are in a "external" file so that they can be reference in multiple examples.
I'd like to be consistent about this and have all examples in files. but this is a much bigger effort at the moment.

Author: jfreire-unity

Packages/com.unity.inputsystem/DocCodeSamples.Tests/InputSystemUIInputModuleAssignActionsExample.cs

@@ -0,0 +1,41 @@
+using UnityEngine;
+using UnityEngine.InputSystem.UI;
+using UnityEngine.Serialization;
+
+namespace DocCodeSamples.Tests
+{
+    internal class InputSystemUIInputModuleAssignActionsExample : MonoBehaviour
+    {
+        // Reference to the InputSystemUIInputModule component, needs to be provided in the Inspector
+        [FormerlySerializedAs("inputModule")]
+        public InputSystemUIInputModule uiModule;
+
+        void Start()
+        {
+            // Assign default actions
+            AssignActions();
+        }
+
+        void AssignActions()
+        {
+            if (uiModule != null)
+                uiModule.AssignDefaultActions();
+            else
+                Debug.LogError("InputSystemUIInputModule not found.");
+        }
+
+        void UnassignActions()
+        {
+            if (uiModule != null)
+                uiModule.UnassignActions();
+            else
+                Debug.LogError("InputSystemUIInputModule not found.");
+        }
+
+        void OnDestroy()
+        {
+            // Unassign actions when the object is destroyed
+            UnassignActions();
+        }
+    }
+}