archwiki: add DarkMode extension

The DarkMode extension should enable a toggle on the wiki. This merge request is intended to coincide with the corresponding PR on the archwiki repository on GitHub.

See the following:

• https://www.mediawiki.org/wiki/Extension:DarkMode
• https://bbs.archlinux.org/viewtopic.php?id=281951
• https://github.com/archlinux/archwiki/pull/59 - previous attempt by killertofus (stalled)