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ARROW-16018: [Doc][Python] Run doctests on Python docstring examples (--doctest-modules)

A series of 3 PRs add `doctest` functionality to ensure that docstring examples are actually correct (and keep being correct).

- [x] Add `--doctest-module`
- [x] Add `--doctest-cython` https://github.com/apache/arrow/pull/13204
- [x] Create a CI job https://github.com/apache/arrow/pull/13216

This PR can be tested with `pytest --doctest-modules python/pyarrow`.

Closes #13199 from AlenkaF/ARROW-16018

Lead-authored-by: Alenka Frim <frim.alenka@gmail.com>
Co-authored-by: Alenka Frim <AlenkaF@users.noreply.github.com>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>