Make right “On this page” box scrollable

[couet]

Make right “On this page” box scrollable, somehow; see e.g. ROOT Primer

[Axel-Naumann]

I think "scollability" is good enough as is: it always focuses on the currently visible section of a page.

Instead we might want to consider expanding the subsections on click, as suggested by @claireguyot That will reduce the need to scroll significantly.