The existing section about `sdist` in `userguide/distribution` seems to
have duplicated content when compared to another section in
`userguide/miscellaneous`. We can combine these two sections and
simplify the documentation.

Changes:

- Consolidate text about `sdist` contents and MANIFEST.in into a single
  place.
- De-emphasize ``distutils`` and the history of ``setuptools``
  (new users might not know about it, which would make them less confuse).

This is a continuation of the update effort to de-emphasize `distutils`
and make the documentation more consistent.

The main targets of the changes are the files
`docs/userguide/miscellaneous` and `docs/userguide/extension`.

Changes:

- Extracted text about automatic resource extraction and the zip-safe flag
  from `userguide/miscellaneous` to `deprecated/resource_extraction` and
  `deprecated/zip_safe`.
  - These configuration parameters are commonly associated with
    ``eggs``/``easy_install``/``pkg_resources``, and therefore are
    obsolete. Leaving them around in the main parts of the documentation
    just confuses users.
  - The text in the new files were updated.
- Extracted text about additional metadata from
  `userguide/miscellaneous` into the existing `userguide/extension`
  document.
- Updated `userguide/extension` to better reflect the status of the
  setuptools project.
  The text was also changed to explain a little bit more what is the
  relationship between ``setuptools`` and ``distutils``.
- Removed `userguide/functionalities_rewrite`.
  This file was virtually empty and not bringing any extra value to the
  docs.

Co-authored-by: Steven Silvester <steven.silvester@ieee.org>