Document scene unique nodes (#5831)

* Document scene unique nodes

* Apply suggestions from code review

Co-authored-by: Hugo Locurcio <hugo.locurcio@hugo.pro>
(cherry picked from commit fd545c7a3e)

tutorials/scripting/index.rst

@@ -51,3 +51,4 @@ below will help you make the most of Godot.
    resources
    singletons_autoload
    scene_tree
+   scene_unique_nodes

tutorials/scripting/scene_unique_nodes.rst

@@ -0,0 +1,36 @@
+.. _doc_scene_unique_nodes:
+
+Scene Unique Nodes
+==================
+
+Introduction
+------------
+
+There are times in a project where a node needs to be called
+from a script. However, its position in the tree might change
+over time as adjustments are made to a scene, such as a
+button in a UI scene.
+
+In situations like this, a node can be turned into a scene
+unique node to avoid having to update a script every time
+its path is changed.
+
+Creating and using them
+-----------------------
+
+In the Scene tree dock, right-click on a node and select
+**Access as Scene Unique Name** in the context menu.
+
+.. image:: img/unique_name.png
+
+After checking this, the node will now have a percent symbol (**%**) next
+to its name in the scene tree:
+
+.. image:: img/percent.png
+
+To use a unique node in a script, use the ``%`` symbol and the node's
+name in the path for ``get_node()``. For example:
+
+.. tabs::
+ .. code-tab:: gdscript GDScript
+    get_node("%RedButton").text = "Hello"