Add ability to collapse sections below headings

Longer topics (aka tl;dr) would be much easier for readers to scan if we could collapse chunks of content under their headings (so long as the headings clearly indicated the hidden content).

This is probably something we'd want to do selectively (i.e. not the default for all pages). It would require adding some JavaScript, either custom written or adapted from some of the code available on the web, and therefore would be best suited to the proposed help.drupal.org subsite.