nixos/doc/rl: mention added module Kimai

nixos/doc/manual/release-notes/rl-2505.section.md

@@ -12,7 +12,7 @@
 
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+- [Kimai](https://www.kimai.org/), a web-based multi-user time-tracking application. Available as [services.kimai](option.html#opt-services.kimai).
 
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