The service pages for retired services all contain a mention of being
retired, but each one is different.

The mention in the gitolite page is clearer than the other ones, with
the section named RETIRED in all caps makes it easier to see as soon as
one opens the document. So we should standardize on that way of marking
service pages as retired.

service/dangerzone.md

 The "dangerzone" service was a documentation sanitization system based
 on the [Dangerzone][] project, using [Nextcloud][] as a frontend.
 
+# RETIRED
+
 It was retired in 2025 because users had moved to other tools, see
 [TPA-RFC-78][].