Reword intro do custom_drawing_in_2d

The wording felt very off from the other tutorials.
This bring it more in line, using a simple 'Introduction' as first section.

tutorials/2d/custom_drawing_in_2d.rst

@@ -3,34 +3,30 @@
 Custom drawing in 2D
 ====================
 
-Why?
-----
+Introduction
+------------
 
 Godot has nodes to draw sprites, polygons, particles, and all sorts of
 stuff. For most cases, this is enough. If there's no node to draw something specific
 you need, you can make any 2D node (for example, :ref:`Control <class_Control>` or
 :ref:`Node2D <class_Node2D>` based) draw custom commands.
 
-But...
-------
+Custom drawing in a 2D node is *really* useful. Here are some use cases:
 
-Custom drawing manually in a node is *really* useful. Here are some
-examples why:
-
--  Drawing shapes or logic that is not handled by nodes (example: making
-   a node that draws a circle, an image with trails, a special kind of
-   animated polygon, etc).
--  Visualizations that are not that compatible with nodes: (example: a
-   tetris board). The tetris example uses a custom draw function to draw
-   the blocks.
+-  Drawing shapes or logic that existing nodes can't do, such as an image
+   with trails or a special animated polygon.
+-  Visualizations that are not that compatible with nodes, such as a
+   tetris board. (The tetris example uses a custom draw function to draw
+   the blocks.)
 -  Drawing a large number of simple objects. Custom drawing avoids the
-   overhead of using nodes which makes it less memory intensive and
-   potentially faster.
+   overhead of using a large number of nodes, possibly lowering memory
+   usage and improving performance.
 -  Making a custom UI control. There are plenty of controls available,
-   but it's easy to run into the need to make a new, custom one.
+   but when you have unusual needs, you will likely need a custom
+   control.
 
-OK, how?
---------
+Drawing
+-------
 
 Add a script to any :ref:`CanvasItem <class_CanvasItem>`
 derived node, like :ref:`Control <class_Control>` or
@@ -62,7 +58,7 @@ The ``_draw()`` function is only called once, and then the draw commands
 are cached and remembered, so further calls are unnecessary.
 
 If re-drawing is required because a state or something else changed,
-simply call :ref:`CanvasItem.update() <class_CanvasItem_method_update>`
+call :ref:`CanvasItem.update() <class_CanvasItem_method_update>`
 in that same node and a new ``_draw()`` call will happen.
 
 Here is a little more complex example, a texture variable that will be