Collapsible tables 2

The code on Collapsible tables 3 does the same thing but is better, so please use it instead.

Section heading
CSS used (in MediaWiki:Common.css)

Uses absolute positioning to put the collapsible toggle in a specific location, but only for a single div that has id="custom-collapsible-div".

Absolute positioning is a bit of a hack, and this set up will not work well for multiple sections/pages. This is a kind of thing that's only designed to work once, in a very specific case.