Fix navigation.tabs for non-leaf pages

Previously, when using navigation.tabs on non-leaf pages, the integrated
TOC used for "layered" (mobile) navigation would incorrectly display as
well. The fix in 87d87f9 was was not
sufficient due to a CSS precedence issue, which is now corrected by
using `!important`.

src/assets/stylesheets/main/layout/_nav.scss

@@ -453,7 +453,7 @@
     // sphinx-immaterial: hide nested nav items of current page, since
     // they are redundant with integrated toc.
     &__current-nested {
-      display: none;
+      display: none !important;
     }
 
     // Show link to table of contents
@@ -514,7 +514,7 @@
 
     // sphinx-immaterial: hide integreated toc, since it is redundant with any nested items.
     &__current-toc {
-      display: none;
+      display: none !important;
     }
 
     // Navigation title