I believe the correct `setup.cfg` packaging option is `find_namespace:`, not `find_namespaces:`
I can't find any other references to `find_namespaces` in the docs or repository.

Using `find_namespaces:` causes packaging to fail. Example to reproduce:
```
.
├── pyproject.toml
├── setup.cfg
└── src
    ├── pkg1
    │   └── namespace
    │       └── __init__.py
    └── pkg2
        └── __init__.py
```

Contents of `pyproject.toml`:
```
[build-system]
requires = ["setuptools"]
build-backend = "setuptools.build_meta"
```

Contents of `setup.cfg`:
```
[metadata]
name = pkg_test

[options]
packages = find_namespaces:
package_dir =
    =src

[options.packages.find]
where = src
```

```
$ pip install .
> ...
> error: package directory 'src/find_namespaces:' does not exist
> ...
```

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`