Add tip explaining how to use ikiwiki as a 404 handler

diff --git a/doc/tips/apache_404_handler.mdwn b/doc/tips/apache_404_handler.mdwn
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+[[!meta title="Apache 404 handler"]]
+
+Sufficiently recent versions of IkiWiki can be used as an Apache 404 handler,
+to give the behaviour of various other wiki engines where visiting a
+nonexistent page provides you with a link to create it.
+
+To achieve this, put something like this in the wiki's Apache configuration
+file:
+
+    ErrorDocument 404 /cgi-bin/ikiwiki.cgi?do=goto