Add CHANGELOG entry for issue #35 under 1.4.0

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: RobertoPrevato

CHANGELOG.md

@@ -13,6 +13,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Fix [#30](https://github.com/Neoteroi/essentials-openapi/issues/30): raise an error
   when trying to generate output from an older Swagger v2 specification file (these were
   never supported as there was never a `/mk/v2/` namespace, intentionally).
+- Fix [#35](https://github.com/Neoteroi/essentials-openapi/issues/35): group response
+  codes in a tab group on MkDocs output, reported by @Andre601.
 
 ## [1.3.0] - 2025-11-19