doc: use a details tag for completed initiatves

The list of completed initiatives is longer and visually more noticeable
than the list of current initiatives, but the current initiatives are
likely the ones of interest. The completed initiatives list is only
going to get longer and longer, so let's put it in a details tag.

PR-URL: https://github.com/nodejs/node/pull/39416


Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>