We have an "In this article" menu for in-page navigation to the right of each page. However, on mobile it disappears, and there is no user interface element that lets you discover it.

Being on mobile makes it particularly hard to navigate longer pages, so it would benefit from the ability to quickly see the page's main structure and jump around within it.

Example of the in-page navigation disappearing:

Example of how this is handled in Jupyter Book

In the pydata and book themes, the right sidebar pops into a drawer on the right. You can open it with a button, similarly to the left drawer:

Example of how this is handled in Docusaurus

In this case they add a dropdown to the top of each page with in-page links and content.