[Doc] Make main column width larger

[asfimport]

In the Sphinx docs, it is currently a bit frustrating that cell tables quickly wrap around, because the width of the main text column is rather small: https://arrow.apache.org/docs/cpp/compute.html#aggregations

It would be nice to find a way to make the width larger. Perhaps by making the left bar narrower and/or collapsable?

Reporter: Antoine Pitrou / @pitrou

Original Issue Attachments:

• sphinx-doc.png

_{Note: This issue was originally created as ARROW-13678. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou: cc @jorisvandenbossche @amol-

[asfimport]

Antoine Pitrou / @pitrou: Attached a screen shot showing the problem on my browser (Firefox, Ubuntu 20.04, default zoom size).