For the `exclude_package_data` example added recently, there were single
backticks around `.gitignore`, `.gitattributes` etc. which leads to the
docs failing to build locally.

Just to make it clear that we can use either one of `package_data` or
`include_package_data` and not just the former.

  - Added `include_package_data`, `package_data` and
    `exclude_package_data` sections to make clear the three options
    provided by Setuptools to manage data files.
  - Added a separate section illustrating the use of a `data`
    subdirectory, after these three sections.
  - Placed the summary of the three options under a Summary section.
  - Changed the levels of the last two sections to match the level of
    the five sections added.
  - Small changes. Changed the wording where appropriate to suit the new
    flow. Changed a paragraph on path separators in glob patterns to a
    Note.