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ARROW-13404: [Doc][Python] Improve PyArrow documentation for new users

* Add link to the cookbooks
* Improve a bit landing page for PyArrow for people that don't already know Arrow
* Add a Getting Started section to introduce people to Arrow and PyArrow

Closes #10999 from amol-/ARROW-13404

Authored-by: Alessandro Molina <amol@turbogears.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>