header: increase invenio menu z-index

Author: kpsherva

invenio_theme/assets/semantic-ui/less/invenio_theme/theme/collections/menu.overrides

@@ -158,7 +158,7 @@
         box-shadow: 0px 0px 7px rgba(0, 0, 0, 0.2);
         background-color: rgb(255, 255, 255);
         height: 100vh;
-        z-index: 3;
+        z-index: 20;
         padding: 2rem 1.5rem 1.5rem 1.5rem;
         overflow-y: scroll;