Restructure the `turtle` documentation

Documentation

Currently, it's a little messy. We have obsolete sections (https://github.com/python/cpython/pull/157846) and several that seem to duplicate each other. Inspired by https://github.com/python/cpython/issues/94635, I will restructure the documentation remove the duplicated content, and to have a clearer order and table of contents than what we have now. A rough plan:

turtle --- Turtle graphics
  (intro prose, merges "Introduction" and "Get started",
   keep the sidebar image, then optional-module include plus the Tk
   note from the how-to, then a "This document includes four main
   sections:" like in sqlite3 documentation)

Tutorial
  Starting a turtle environment
  Basic drawing
  Making algorithmic patterns
  etc.

Reference
  Turtle methods and functions             (current "Methods of RawTurtle/Turtle...")
  Screen methods and functions             (current "Methods of TurtleScreen/Screen...")
  Classes                                  (current "Public classes")
  Exceptions

How-to guides
  How to automatically begin and end filling
  How to use the turtle module namespace
  How to use turtle graphics in a script
  How to use object-oriented turtle graphics
  How to use help                          (moved from "Help and configuration")
  How to configure Screen and Turtles      (moved from "Help and configuration")

Explanation
  The object-oriented interface            (current "Explanation")

turtledemo --- Demo scripts                (unchanged)

Linked PRs

• gh-157846
• gh-157849
• gh-157850
• gh-157851
• gh-157854
• gh-157868