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ARROW-9570: [Doc] Clean up sphinx sidebar

Saying "Arrow" is redundant and consumes unnecessary screen real estate, leading to:

![image](https://user-images.githubusercontent.com/2975928/88416853-ca88b700-cd95-11ea-82f8-fe67d80fdf66.png)

Closes #7841 from nealrichardson/sphinx-sidebar

Authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>