Sections in sidebar let's you categorize different pages in your documentation site.
You can have different categories for your docs such as INTRODUCTION, API, GUIDES, SCREENCASTS, FAQ, CONTRIBUTING, SUPPORT, etc..
The documentation that you are looking at also has different pages categorized into INTRO, VS OTHERS, FEATURES and USAGE in the sidebar.
Adding a category heading is very easy in Hyperdocs. Simply add a new new file that starts with - (that's a hyphen followed by a space) and then the category name. For example, - INTRO.
Here's the `_sidebar.txt` file for Hyperdocs with categories: