Extension:SectionHide

The SectionHide extension adds a [hide] link wherever there is a section heading [edit] link. This link will hide all content between that section header and the next section header of the same level, changing itself to read [show] in the process. Clicking [show] will restore the section to view.

SectionHide.i18n.php
Only the English to get started, add languages as you need them please.

Mediawiki:Common.js
Add this here, or in individual skins

Author Notes
Several similar extensions were created in the past, but they are not maintained and didn't do quite what I needed.