Center align the text and icons in documentation index

[TomAugspurger]

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

cc @jorisvandenbossche

[jorisvandenbossche]

Hmm, indeed. Looking at it with browser developer tools, for some reason there is padding on the left but no on the right in the card-img-top div.

Removing

.intro-card .card-img-top {
  margin: 10px;
}

from pandas.css seems to help (it makes there is a bit less whitespace above the icons, but not sure that is a problem)

cc @stijnvanhoey

[TomAugspurger]

I think it looks fine without the margin.