docs: hide 'back-to-main-menu' entry in narrow menu

This hides the 'back to main menu' button from the narrowed site menu.
There is no main menu to speak of, so having it take you back to
'unleash enterprise' and the GitHub icon seems unnecessary.

website/src/css/custom.css

@@ -29,7 +29,6 @@
     --navbar-link-color: #122d33;
 }
 
-
 footer {
     --ifm-footer-link-hover-color: var(--ifm-footer-link-color);
 }
@@ -38,6 +37,11 @@ footer {
     font-size: 2.5rem !important;
 }
 
+.navbar-sidebar__back {
+    /* hide the 'back to main menu' item in the narrow menu. */
+    display: none;
+}
+
 li.theme-doc-sidebar-item-category-level-1 > div::before {
     width: 0.3em;
     height: 100%;