add public code restructure warning, to make it clear configuration changes are required as part of a 5.1 upgrade

Author: mattporritt

general/releases/5.1.md

@@ -12,6 +12,15 @@ import { ReleaseNoteIntro } from '@site/src/components/ReleaseInformation';
 
 <ReleaseNoteIntro releaseName={frontMatter.moodleVersion} />
 
+:::warning Important 5.1 upgrade notice
+The directory structure of Moodle files has changed in 5.1.
+A new **"public"** folder has been introduced to contain all web accessible files.
+Configuration and other sensitive files are now stored outside this folder for better security.
+
+You will need to reconfigure your web server to accommodate this change as part of the upgrade process.
+Please review the configuration information, see [Code Restructure - Moodle Developer Resources](/docs/5.1/guides/restructure) prior to beginning the upgrade process 5.1.
+:::
+
 If you are upgrading from a previous version, please see [Upgrading](https://docs.moodle.org/en/Upgrading) in the user docs.
 
 ## Server requirements