Make sections foldable (collapsible headings)

Spoilers/foldable details sections are tracked separately in https://github.com/nextcloud/text/issues/3646

This ticket remains about making existing sections like headings foldable.

Initial report

Is your feature request related to a problem? Please describe.
For large pages, it is hard to have a vision of the structure, or to navigate within the page to find the right section / paragraph.

Describe the solution you'd like
It would be great to be able to collapse (fold) and open (unfold) sections, thanks to a small arrow on the left. Sections would be based on headers (H1, H2, ...), with subsections that would also be collapsible (foldable). In addition, a button on the top would fold/unfold all.

Describe alternatives you've considered
Having a menu on the top or on the right sidebar (also with collapsible sections)

Additional context

• An example is described in the following link: https://gist.github.com/pierrejoubert73/902cc94d79424356a8d20be2b382e1ab
• Microsoft describes its own solution here: https://support.microsoft.com/en-us/office/video-collapsible-headings-c7b03e92-a397-479d-b989-57393daf2d65
• This other issue mentions a collapsible section for a specific file type: https://github.com/nextcloud/text/issues/145

No implementation file, test, or entry point is named. Start by locating the heading and editor handling in this JavaScript Tiptap/ProseMirror project, then clarify nested folding and fold/unfold-all behavior; done means existing headings can collapse and expand without including the separately tracked details-section work.