Add ImGui demo window to rendering loop

Introduced a call to `ImGui.ShowDemoWindow()` within the
rendering loop to display the Dear ImGui demo window. This
provides a showcase of various UI elements and features
available in the library. The change is likely intended for
debugging, learning, or demonstrating ImGui capabilities.
The rest of the rendering process remains unchanged.

Author: JunaMeinhold

Examples/ExampleGLFWOpenGL3/Program.cs

@@ -99,6 +99,8 @@
     ImGuiImplGLFW.NewFrame();
     ImGui.NewFrame();
 
+    ImGui.ShowDemoWindow();
+
     ImGui.Render();
 
     GLFW.MakeContextCurrent(window);