Reorganized the User Guide's Table of Contents
Reorganized the User Guide's Table of Contents

This mostly follows the scheme given
[here](https://github.com/pypa/setuptools/discussions/3400#discussioncomment-3003334),
with the following exceptions:
 - "Controlling files in the distribution" is kept not immediately after
   the "Package Discovery" page, but rather towards the end, because a
   lot of the material in that page overlaps with material given in the
   "Package Discovery" and "Data Files" pages. Therefore, it seems to me
   that this page should be read _after_ the other two pages have been
   read.
 - "Development Mode" is kept not towards the end but introduced close
   to the beginning, because readers might want to start using it early
   on in their projects.
 - "Building Extension Modules" is kept immediately after the "Entry
   Points" and "Data Files" pages, since it is the last major topic we
   would like to discuss. The two topics that follow ("Specifying your
   Project's Version" and "Controlling Files in the Distribution") are
   lighter topics.
 - Have retained the pages "Extending or Customizing Setuptools",
   "Configuring Setuptools using setup.cfg files" and "Configuring
   Setuptools using pyproject.toml files" for now.

pypa/setuptools,pypa/setuptools,pypa/setuptools