How to add a sidebar to an article
Click on the Layout Options icon at the top right of the navigation.

Add a sidebar on the left or right side of the main content.

Within this panel, you have the option to adjust the sidebar's width, decide its "sticky" state (whether it stays fixed at the top while scrolling or not), and insert a dividing line between the sidebar and the rest of the content.
How to add content to the sidebar
After you enable the sidebar, it will be visible in your canvas and you can add any component to it using the slash character or the plus icon.
Typically, a sidebar can be used to display an author profile or an internal navigation menu.
To add a navigation menu, select the "Page overview" option.

This will create a clickable menu that includes all the H2 and H3 titles in your article.