Right-side navigation picking up Headings in Text Web Part

Hi all,

First post (and I really have looked everywhere before submitting myself to this question).

How does one go about the right-hand in page navigation, seen throughout these pages... 

https://docs.microsoft.com/en-us/sharepoint/introduction

...as the "In this article" navigation?

I'm assuming it's some kind of web-part, that is able to pick up on Heading styles in the Text web part that carries the body of text for each page.

And for a bonus point, can I this section to pick up two levels of Heading Style?

Any help or guidance would be much appreciated.

Cheers,

Jim