Create a file named offline.txt in your data folder.
The content is plain text and will be shown to visitors that will attempt to use your site.
Note that not even the site admin can log in when it is in maintenance mode.
If you delete the file (or rename it to !offline.txt, so you can rename it back when needed) then the site becomes available again.
I estimate about 90% of the existing articles in the old wiki still apply to v2.x