How to add more "INFO" tag on top of a page.

Hi, I would like to add more additional information tags like "INFO" tag on top of a page.

The example page as below.

https://www.adsuisse.com/api/docs

In this page, there are menus at the left pane(Introduction, Authenticatetion and more).

Would you please share me how to display an additional information in the left pane.

I'm coding with HTML and Javascript, so it would be helpful to share me en example in that way.

 

Best Regards,