Navigation Panel or HUD

I work with very long documents. Currently it is very difficult to find the right section within the document. It would be great if you could add a navigation panel or HUD which would allow me to jump to a specific section within the document. The navigation panel should display all the headers of the document hierarchically and should allow me to collapse certain levels if needed.

The issue does not name any files, tests, or entry points. Start by locating MacDown’s document editor and Markdown header-rendering components, then determine how a hierarchical navigation panel or HUD could jump to headers and collapse levels; done means all document headers are shown hierarchically and navigation works.

Written by the indexing model from the issue text.