The `ping` directive is supplied by the [[!iki plugins/pinger]] plugin.

This directive allows ikiwiki to be configured to hit a URL each time it
updates the wiki. One way to use this is in conjunction with the [[!iki plugins/pingee]]
plugin to set up a loosely coupled mirror network, or a branched version of
a wiki. By pinging the mirror or branch each time the main wiki changes, it
can be kept up-to-date.

        \[[!ping from="http://mywiki.com/"
        to="http://otherwiki.com/ikiwiki.cgi?do=ping"]]

The "from" parameter must be identical to the url of the wiki that is doing
the pinging. This is used to prevent ping loops.

The "to" parameter is the url to ping. The example shows how to ping
another ikiwiki instance.