How to make a live website "under maintenance"?


I would like to do a major maintenance on a live website. How would I disable the site or show a page which says “Under maintenance”?
In wordpress I can do this via a plugin.


Check out this post for some possibilities.

The easiest way would be to put an index.html file into the kirby folder (provided, your server is configured to prefer .html over .php), which you then rename or remove afterwards.