Unable to create child menu/ sub menu


I am a new user of Jekyl Minimal Mistakes theme. Recently, I am planning to develop a site where there will be sub-menu/ pages for few of the parent menu options. But I am unable to achieve this.

It would be a great help if anyone can help me on this issue. I have attached a screenshot of what I want to achieve. So, if I click on “About”, then the website will display the content of about.html as well as it will show the sub-pages that are available under About page.


Rayhanul Islam Rumel