Center align the text and icons in documentation index
At https://dev.pandas.io/docs/index.html, the alignment is slightly off. Especially notable for User Guide and API reference

![Screen Shot 2020-01-27 at 3 27 16 PM](https://user-images.githubusercontent.com/1312546/73215355-90f22e80-4119-11ea-9f6b-d9bedd1f332b.png)

cc @jorisvandenbossche 

