docs: Add changelog

Author: gadenbuie

CHANGELOG.md

@@ -5,6 +5,12 @@ All notable changes to Shiny for Python 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).
 
+## [UNRELEASED]
+
+## Bug fixes
+
+* Fixed an issue where the `<main>` areas of `page_sidebar()` and `page_navbar()` (with a `sidebar`) were made to be a fillable containers even when `fillable=False`. (#1816)
+
 ## [1.3.0] - 2025-03-03
 
 ### New features