doc: increase z-index of header element

increase the z-index of the header element to make sure that
opened pickers from it (such as the node version picker) are
on top of other content

PR-URL: #57851
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>

Author: dario-piotrowicz

doc/api_assets/style.css

@@ -220,7 +220,6 @@ li.picker-header.expanded > .picker,
 :root:not(.has-js) li.picker-header:focus-within > .picker,
 :root:not(.has-js) li.picker-header:hover > .picker {
   display: block;
-  z-index: 1;
 }
 
 li.picker-header a span {
@@ -821,7 +820,7 @@ kbd {
 .header {
   position: sticky;
   top: -1px;
-  z-index: 1;
+  z-index: 10;
   padding-top: 1rem;
   background-color: var(--color-fill-app);
 }