docs: prepare changelog for release 5.1.0 (#426)

Author: colinscz

docs/changelog.md

@@ -6,6 +6,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 
 ## Upcoming
 
+## 5.1.0 - 2025-10-23
+
 ### Added
 
 - `six-icon`: Support for **Material Symbols** (variable font). Select per instance via the new