Update images in Scenes and Nodes for Godot 3.1 and add extra information (#2822)

getting_started/step_by_step/scenes_and_nodes.rst

@@ -86,7 +86,13 @@ appears. This helps developers manage their projects.
 .. image:: img/project_manager.png
 
 To create a new project, click the "New Project" option. Choose and create a
-path for the project and specify the project name "New Project":
+path for the project and specify the project name.
+
+You can also select the rendering backend at this stage (OpenGL ES 3.0 or OpenGL
+ES 2.0). The advantages and disadvantages of each are listed to help you choose,
+and you can refer to :ref:`doc_gles2_gles3_differences` for more details. Note
+that you can change the backend from the project settings if you change your mind
+later on. For this tutorial either backend is fine.
 
 .. image:: img/create_new_project.png
 
@@ -103,7 +109,30 @@ kitchen, so let's open the refrigerator and add some fresh nodes to the
 project. We'll begin with a "Hello World" message that we'll put on the
 screen.
 
-To do this, press the "New Node" button (which looks like a plus symbol):
+To do this we need to add a Label node. Press the "Add Child Node" button
+at the top left of the scene dock (the icon represents a plus symbol).
+This button is the main way to add new nodes to a scene, and will always
+add the chosen node as a child of the currently selected node (or, in an
+empty scene, as the "root" node).
+
+.. note::
+
+    In an empty scene (without root node), the scene dock shows several
+    options to quickly add a root node to the scene. "2D Scene" adds a
+    Node2D node, "3D Scene" adds a Spatial node, "User Interface" adds a
+    Control node, and "Custom Node" which lets you select any node (so it
+    is equivalent to pressing the "Add Child Node" button). You can also
+    press the star-shaped icon to toggle the display of your favorited
+    nodes.
+
+    Note that these presets are here for convenience and are not mandatory
+    for the different types of scenes. Not every 3D scene needs a Spatial
+    node as its root node, likewise not every GUI or 2D scene needs a Control
+    node or Node2D as their root node.
+
+Now, to add a label node to this scene you can click on the Custom Node
+button or the Add Node button at the top. In scenes that aren't empty you
+use the add node button to create every child node.
 
 .. image:: img/newnode_button.png