Center Hero content (scientific-python#250)

This starts to address the problem where content tends to shift to the
left for short Hero titles.  The solution is not complete, in that
scaling is still a bit odd for certain screen widths, where there is a
notable space between the logo and the hero title.

assets/theme-css/styles.css

@@ -36,7 +36,7 @@ em {
   align-items: center;
   display: flex;
   flex-direction: row;
-  justify-content: space-around;
+  justify-content: center;
   width: 100%;
 }