doc: fix invalid HTML in doc template

Move the script tag for dark mode support inside the body tag.

PR-URL: https://github.com/nodejs/node/pull/36930


Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>