fix: only render secondaryMenu if it should be shown

fixes facebook#10704

Issue was that with keyboard, you could reach the primary menu, while it was made invisible by CSS. By not rendering secondaryMenu based on the secondaryMenuShown logic, , we make sure it isn't accessible by keyboard while it is hidden.

packages/docusaurus-theme-classic/src/theme/Navbar/MobileSidebar/Layout/index.tsx

@@ -19,13 +19,12 @@ export default function NavbarMobileSidebarLayout({
   return (
     <div className="navbar-sidebar">
       {header}
-      <div
-        className={clsx('navbar-sidebar__items', {
-          'navbar-sidebar__items--show-secondary': secondaryMenuShown,
-        })}>
-        <div className="navbar-sidebar__item menu">{primaryMenu}</div>
-        <div className="navbar-sidebar__item menu">{secondaryMenu}</div>
-      </div>
-    </div>
+      <div className={clsx('navbar-sidebar__items')}>
+        {secondaryMenuShown ? (
+          <div className="navbar-sidebar__item menu">{secondaryMenu}</div>
+        ) : (
+          <div className="navbar-sidebar__item menu">{primaryMenu}</div>
+        )}    
+     </div>
   );
 }