sidebar atop header (#1803)

Author: mbostock

src/style/layout.css

@@ -123,7 +123,7 @@ body {
   visibility: hidden;
   font-weight: 500;
   width: calc(272px + var(--observablehq-sidebar-padding-left));
-  z-index: 2;
+  z-index: 3;
   top: 0;
   bottom: 0;
   left: -272px;