DOC: Fix table styling in main docs (#16145)

When switching to nbsphinx, I modified the site's CSS so
that the converted notebook looks decent. This had some
unfortunate changes on tables elsewhere in the notebook.

This change fixes the headers to be left-aligned in the main site,
and right-aligned for the tables generated by `df.style` in the
nbsphinx-converted notebook.

xref #15581

Author: TomAugspurger

doc/source/themes/nature_with_gtoc/static/nature.css_t

@@ -315,7 +315,6 @@ thead {
     vertical-align: bottom;
 }
 tr, th, td {
-    text-align: right;
     vertical-align: middle;
     padding: 0.5em 0.5em;
     line-height: normal;
@@ -326,6 +325,9 @@ tr, th, td {
 th {
     font-weight: bold;
 }
+th.col_heading {
+    text-align: right;
+}
 tbody tr:nth-child(odd) {
     background: #f5f5f5;
 }