Add a page referencing the Control gallery image and demo

This closes #4435.

tutorials/ui/control_node_gallery.rst

@@ -0,0 +1,9 @@
+Control node gallery
+====================
+
+Here is a list of common Control nodes with their name next to them:
+
+.. image:: /img/control_gallery.png
+
+The Control Gallery demo pictured above can be found
+`on GitHub <https://github.com/godotengine/godot-demo-projects/tree/master/gui/control_gallery>`__.

tutorials/ui/index.rst

@@ -10,3 +10,4 @@ User Interface (UI)
    size_and_anchors
    gui_containers
    bbcode_in_richtextlabel
+   control_node_gallery