[SPARK-33796][DOCS][FOLLOWUP] Tweak the width of left-menu of Spark SQL Guide
[SPARK-33796][DOCS][FOLLOWUP] Tweak the width of left-menu of Spark SQL Guide

### What changes were proposed in this pull request?

This PR tweaks the width of left-menu of Spark SQL Guide.
When I view the Spark SQL Guide with browsers on macOS, the title `Spark SQL Guide` looks prettily.
But I often use Pop!_OS, an Ubuntu variant, and the title is overlapped with browsers on it.
![spark-sql-guide-layout-before](https://user-images.githubusercontent.com/4736016/104002743-d56cc200-51e4-11eb-9e3a-28abcd46e0bf.png)

After this change, the title is no longer overlapped.
![spark-sql-guide-layout-after](https://user-images.githubusercontent.com/4736016/104002847-f9c89e80-51e4-11eb-85c0-01d69cee46b7.png)

### Why are the changes needed?

For the pretty layout.

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

Built the document with `cd docs && SKIP_API=1 jekyll build` and confirmed the layout.

Closes #31091 from sarutak/modify-layout-sparksql-guide.

Authored-by: Kousuke Saruta <7d45abe39fd242ee2f6575da28f13d66568418b3@oss.nttdata.com>
Signed-off-by: Sean Owen <d519fa33a62684a61ab04b24d26f83452d51b34c@gmail.com>

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