Update CHANGELOG.md for version 3.21.1

Author: github-actions[bot]

CHANGELOG.md

@@ -5,6 +5,13 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [3.21.1](https://github.com/Parsely/wp-parsely/compare/3.21.0...3.21.1) - 2025-10-13
+
+### Fixed
+
+- Settings page: Fix Headline Testing documentation link ([#3753](https://github.com/Parsely/wp-parsely/pull/3753))
+
+
 ## [3.21.0](https://github.com/Parsely/wp-parsely/compare/3.20.8...3.21.0) - 2025-10-13
 
 ### Added