You can create nested menus in Bitrix24 Knowledge Base by creating a standard menu and dragging the menu items.
Create a standard menu
To add a new menu, click Add block > Sidebar and select a suitable vertical menu.
Then click the cogwheel button > edit.
If it's a new menu, remove menu items that you don't need and add new ones by clicking the + button.
When you use this method of adding menu items, the Knowledge Base page for this menu item is generated automatically.
Create a nested menu
To create a nested menu, you need to create a standard menu item and drag it where you want it.
You can also change the order of menu items: