docs: improve algolia dropdown menu shadow effect (#6845)

Author: kovsu

site/src/theme/static/docsearch.less

@@ -1,7 +1,7 @@
 .algolia-autocomplete {
   .ds-dropdown-menu {
     border: none;
-    box-shadow: var(--box-shadow-base);
+    box-shadow: var(--box-shadow);
 
     [class^='ds-dataset-'] {
       background: var(--component-background);