Content-wise, the move is done. I've done a few checks on the content to see if the structure still holds, translations are enabled on all pages, the use of partitions is sufficiently consistent for each architecture, and so on. The result can be seen on the gentoo handbook main page, from which the various architectural handbooks are linked.
I sent a sort-of announcement to the gentoo-project mailinglist (which also includes the motivation of the move). If there are no objections, I will update the current handbooks to link to the wiki ones, as well as update the links on the website (and in wiki articles) to point to the wiki.