roadmap: 2x2 grid instead of 3-per-row

Previous CSS used 3-column grid which left the 4th roadmap item
(Programmatic API access) orphaned on its own row after the 6->4
restructure. Switch to a 2-column grid with max-width 900px so the
four items render as a balanced 2x2 block.

Mobile breakpoint (<=768px) stays single-column.

Author: annefou

static/css/science-live.css

@@ -3021,9 +3021,10 @@ section[id] {
 
 .sl-roadmap-grid {
     display: grid;
-    grid-template-columns: repeat(3, 1fr);
+    grid-template-columns: repeat(2, 1fr);
     gap: 1.5rem;
-    margin-top: 2rem;
+    margin: 2rem auto 0;
+    max-width: 900px;
 }
 
 .sl-roadmap-item {