upgrade: Add link to v2.4 example config

Author: sirainen

docs/installation/upgrade/2.3-to-2.4.md

@@ -23,6 +23,12 @@ If you are doing in-place upgrade, ensure that you first upgrade to latest
 Alternatively, you can [[link,migrating_mailboxes,migrate]] your data to new
 setup.
 
+## Example Configuration
+
+The old v2.3 example configuration has been converted to v2.4 once. It's not
+being kept up-to-date afterwards, but it can be helpful:
+https://github.com/dovecot/tools/blob/main/dovecot-2.4.0-example-config.tar.gz
+
 ## Dovecot CE
 
 ### Configuration Changes