Remove thicker border on table thead elements

Use the new .table-group-divider to create your own dividers as desired. Would love to find a better way to handle border-color for this, but for now, this is at least opt-in.

I've applied it by default in another way for our docs tables to help differentiate our content vs our components.

Fixes #35342

site/assets/scss/_content.scss

@@ -45,6 +45,10 @@
       }
     }
 
+    thead {
+      border-bottom: 2px solid currentColor;
+    }
+
     th,
     td {
       &:first-child {