add new selector (#1078)

Author: ethanpalm

settings/custom-scripts.mdx

@@ -137,6 +137,7 @@ Mintlify has a set of common identifiers and selectors to help you tag important
   - SidebarGroup: `sidebar-group`
   - SidebarGroupIcon: `sidebar-group-icon`
   - SidebarGroupHeader: `sidebar-group-header`
+  - SidebarNavGroupDivider: `sidebar-nav-group-divider`
   - SidebarTitle: `sidebar-title`
   - Step: `step`
   - Steps: `steps`