Add the basics of a turtle graphics tutorial

The tutorial follows the practices outlined in
https://diataxis.fr/tutorials/. It uses the functional turtle
interface, and doesn't mention the object-oriented interface (which
would be a good next step).

Existing sections are now clearly labelled "Explanation" and
"Reference" to clarify their scope.

Author: evildmp

Doc/includes/turtle-star.py

@@ -24,6 +24,14 @@ introduced in Logo <https://en.wikipedia.org/wiki/Turtle_
 (robot)>`_, developed by Wally Feurzeig, Seymour Papert and Cynthia Solomon
 in 1967.
 
+.. sidebar:: Turtle star
+
+   Turtle can draw intricate shapes using programs that repeat simple
+   moves.
+
+   .. image:: turtle-star.*
+      :align: center
+
 In Python, turtle graphics provides a representation of a physical "turtle"
 (a little robot with a pen) that draws on a sheet of paper on the floor.
 
@@ -38,26 +46,115 @@ graphical output it can be a way to do that without the overhead of
 introducing more complex or external libraries into their work.
 
 
-Get started
-===========
+Tutorial
+========
 
-Imagine a robotic turtle starting at (0, 0) in the x-y plane.  After an ``import turtle``, give it the
-command ``turtle.forward(15)``, and it moves (on-screen!) 15 pixels in the
-direction it is facing, drawing a line as it moves.  Give it the command
-``turtle.right(25)``, and it rotates in-place 25 degrees clockwise.
+New users should start here. In this tutorial we'll explore some of the
+basics of turtle drawing.
 
-.. sidebar:: Turtle star
 
-   Turtle can draw intricate shapes using programs that repeat simple
-   moves.
+Starting a turtle environment
+-----------------------------
 
-   .. image:: turtle-star.*
-      :align: center
+In a Python shell, import all the objects of the ``turtle`` module::
+
+    from turtle import *
+
+If you run into a ``No module named '_tkinter'`` error, you'll have to
+install the :mod:`Tk interface package <tkinter>` on your system.
+
+
+Basic drawing
+-------------
+
+Send the turtle forward 100 steps::
+
+   forward(100)
+
+You should see (most likely, in a new window on your display) a line
+drawn by the turtle, heading East. Change the direction of the turtle,
+so that it turns 120 degrees left (anti-clockwise)::
+
+   left(120)
+
+Let's continue by drawing a triangle::
+
+   forward(100)
+   left(120)
+   forward(100)
+
+Notice how the turtle, represented by an arrow, points in different
+directions as you steer it.
 
-   .. literalinclude:: ../includes/turtle-star.py
+Experiment with those commands, and also with ``backward()`` and
+``right()``.
 
-By combining together these and similar commands, intricate shapes and pictures
-can easily be drawn.
+Try changing the color - for example, ``color('blue')`` - and
+width of the line - for example, ``width(3)``.
+
+Send your turtle back to its starting-point (useful if it has disappeared
+off-screen)::
+
+   home()
+
+The home position is at the center of the turtle's screen. If you ever need to
+know them, get the turtle's x-y co-ordinates with::
+
+    pos()
+
+And after a while, it will probably help to clear the window so we can start
+anew::
+
+   clearscreen()
+
+
+Making algorithmic patterns
+---------------------------
+
+Using loops, it's possible to build up geometric patterns::
+
+    steps = 1
+    while steps < 100:
+        for c in ('blue', 'red', 'green'):
+            color(c)
+            forward(steps)
+            right(30)
+            x += 1
+
+
+\ - which of course, are limited only by the imagination!
+
+Try this::
+
+    color('red')
+    fillcolor('yellow')
+
+This means that the turtle will draw red lines, and use yellow to fill
+between the lines. It needs to know to at which point filling should
+begin::
+
+    begin_fill()
+
+Next we'll create a loop::
+
+    while True:
+        forward(200)
+        left(170)
+        if abs(pos()) < 1:
+            break
+
+``abs(pos()) < 1`` is a good way to know when the turtle is back at its
+home position.
+
+Finally, complete the filling::
+
+    end_fill()
+
+This will draw the star shape at the top of this page.
+
+
+Explanation
+===========
 
 The :mod:`turtle` module is an extended reimplementation of the same-named
 module from the Python standard distribution up to version Python 2.5.
@@ -112,8 +209,8 @@ To use multiple turtles on a screen one has to use the object-oriented interface
    omitted here.
 
 
-Overview of available Turtle and Screen methods
-=================================================
+Turtle graphics reference
+=========================
 
 Turtle methods
 --------------