This patch helps clarify the purpose, value and scope of the turtle
graphics module, by rewriting the introduction according to the
pattern:

* a single sentence that says **what the product (or thing) is**,
  as succinctly as possible
* a brief description of **what it does**
* a statement of its purpose or **the problem it solves**
* a note about **whom it is useful for**

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.