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**

Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>