The node a page is being created for.
This template is a stub, called by html.page. It is called inside the main element, before the div.page element. The main element uses a horizontal flexbox by default. You can override this template to provide sidebar content. Note that there is only one extension point for sidebar content, and it is always placed before the main content in document order. To create a sidebar on the right, output the element here, then adjust the order CSS property for that element to display it after the main element.
This template is intended for completely custom sidebars. You can also use html.sidebar to output sidebars with stock components.