docs: add changelog and ugrade note

Author: kenjis

user_guide_src/source/changelogs/v4.4.0.rst

@@ -64,6 +64,14 @@ Autoloader
 Previously, CodeIgniter's autoloader allowed loading class names ending with the `.php` extension. This means instantiating objects like `new Foo.php()` was possible
 and would instantiate as `new Foo()`. Since `Foo.php` is an invalid class name, this behavior of the autoloader is changed. Now, instantiating such classes would fail.
 
+.. _v440-codeigniter-and-exit:
+
+CodeIgniter and exit()
+----------------------
+
+The ``CodeIgniter::run()`` method no longer calls ``exit(EXIT_SUCCESS)``. The
+exit call is moved to **public/index.php**.
+
 .. _v440-interface-changes:
 
 Interface Changes

user_guide_src/source/installation/upgrade_440.rst

@@ -117,6 +117,24 @@ match the new array structure.
 Mandatory File Changes
 **********************
 
+index.php
+=========
+
+The following file received significant changes and
+**you must merge the updated versions** with your application:
+
+- ``public/index.php`` (see also :ref:`v440-codeigniter-and-exit`)
+
+.. important:: If you don't update the above file, CodeIgniter will not work
+    properly after running ``composer update``.
+
+    The upgrade procedure, for example, is as follows:
+
+    .. code-block:: console
+
+        composer update
+        cp vendor/codeigniter4/framework/public/index.php public/index.php
+
 Config Files
 ============