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

Adding `--doctest-cython` functionality which will be run on the CI with a follow-up PR.

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

Closes #13204 from AlenkaF/ARROW-16018-doctest-cython

Authored-by: Alenka Frim <frim.alenka@gmail.com>
Signed-off-by: Alessandro Molina <amol@turbogears.org>