Add 7.2.3 Release notes (#398)

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<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Documentation**
* Added release notes for Unraid OS 7.2.3, documenting security and
bugfix updates across Storage, webGUI, and system components. Includes
Samba library updates, known issues, and upgrade/rollback guidance.

<sub>✏️ Tip: You can customize this high-level summary in your review
settings.</sub>

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: ljm42

docs/unraid-os/release-notes/7.2.3.md

@@ -0,0 +1,46 @@
+# Version 7.2.3 2025-12-18
+
+This is a security and bugfix release, recommended for all users.
+
+## Upgrading
+
+For step-by-step instructions, see [Upgrading Unraid](../../unraid-os/system-administration/maintain-and-update/upgrading-unraid.mdx). Questions about your [license](../../unraid-os/troubleshooting/licensing-faq.mdx#license-types--features)?
+
+### Known issues
+
+For other known issues, see the [7.2.2 release notes](7.2.2.md#known-issues).
+
+### Rolling back
+
+If rolling back earlier than 7.2.2, also see the [7.2.2 release notes](7.2.2.md#rolling-back).
+
+## Changes vs. [7.2.2](7.2.2.md)
+
+### Storage
+
+- Fix: Upgraded Samba to resolve issues with Time Machine
+- Fix: Precleared disk signature only detected after a reboot
+
+### webGUI
+
+- Fix: Regression in how gradients are applied over background images in header
+- Fix: Notification text is sometimes shown in wrong colors, making it hard to read
+- Fix: Potential XSS issue when testing SMTP config on ***Settings → Notification Settings → SMTP Settings***
+
+### Misc
+
+- Fix: DNS settings could be overwritten by other devices on the network (disable avahidnsconfd)
+- Fix: Readme not populating on Docker XML templates
+- Fix: Missing novnc files causing warnings in syslog
+
+### Unraid API
+
+- dynamix.unraid.net 4.28.2 - [see changes](https://github.com/unraid/api/releases)
+
+### Linux kernel
+
+- version 6.12.54-Unraid (no change)
+
+### Base distro updates
+
+- samba: version 4.23.4