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.

Also add a not about modern ways of generating ``sdists`` and ``wheel``.

- Improve docs by avoiding confusion with distutils
- Update dependency management docs
- Update userguide on miscellaneous and extension

The existing docs seem to assume that the user is familiar with the
history of packaging in the Python ecosystem (or at least know what is
`distutils`).

Since that is not always the case and that `distutils` is in the process
of being adopted by `setuptools`, the documentation should be changed
to minimize mentions to `distutils` and avoid expecting too much
knowledge from the users.

The benefit of this approach is that it can make the docs more
accessible and easier to understand.

Changes:

- Modify landing page to clarify what `setuptools` does (making it more
  clear to understand for beginners).
- Remove mentions to `distutils`, `transition to PEP 517` from
  `userguide/index`.
  - Instead the text is changed to have a more "introductory" tone.
- Remove mentions to `distutils` from the Quickstart.
- Remove `python2` from the intersphinx mapping - it was causing trouble
  redirecting glossary terms to Python2 docs, instead of Python3.
- Modify documentation about development mode to be more aligned with
  current practices (i.e. using `pip install -e .`)
  - In this process all documentation about running `setuptools` commands
    in `distutils` projects was moved to a new file in `docs/deprecated/running_commands.rst`