docs: add changelog and upgrading guide

Author: kenjis

user_guide_src/source/changelogs/v4.2.7.rst

@@ -12,7 +12,7 @@ Release Date: Unreleased
 BREAKING
 ********
 
-none.
+-  ``Time::__toStrong()`` is now locale-independent. It returns database-compatible strings like '2022-09-07 12:00:00' in any locale.
 
 Enhancements
 ************

user_guide_src/source/installation/upgrade_427.rst

@@ -0,0 +1,33 @@
+#############################
+Upgrading from 4.2.6 to 4.2.7
+#############################
+
+Please refer to the upgrade instructions corresponding to your installation method.
+
+- :ref:`Composer Installation App Starter Upgrading <app-starter-upgrading>`
+- :ref:`Composer Installation Adding CodeIgniter4 to an Existing Project Upgrading <adding-codeigniter4-upgrading>`
+- :ref:`Manual Installation Upgrading <installing-manual-upgrading>`
+
+.. contents::
+    :local:
+    :depth: 2
+
+Breaking Changes
+****************
+
+-  ``Time::__toStrong()`` is now locale-independent. It returns database-compatible strings like '2022-09-07 12:00:00' in any locale. Most locales are not affected by this change. But in a few locales like `ar`, `fa`, ``Time::__toStrong()`` (or ``(string) $time`` or implicit casting to a string) no longer returns a localized datetime string. if you want to get a localized datetime string, use :ref:`Time::toDateTimeString() <time-todatetimestring>` instead.
+
+Project Files
+*************
+
+A few files in the **project space** (root, app, public, writable) received cosmetic updates.
+You need not touch these files at all. There are some third-party CodeIgniter modules available
+to assist with merging changes to the project space: `Explore on Packagist <https://packagist.org/explore/?query=codeigniter4%20updates>`_.
+
+All Changes
+===========
+
+This is a list of all files in the **project space** that received changes;
+many will be simple comments or formatting that have no effect on the runtime:
+
+*

user_guide_src/source/installation/upgrading.rst

@@ -16,6 +16,7 @@ See also :doc:`./backward_compatibility_notes`.
 
     backward_compatibility_notes
 
+    upgrade_427
     upgrade_426
     upgrade_425
     upgrade_423

user_guide_src/source/libraries/time.rst

@@ -149,6 +149,8 @@ A full listing of values can be found `here <https://unicode-org.github.io/icu-d
 
 .. literalinclude:: time/015.php
 
+.. _time-todatetimestring:
+
 toDateTimeString()
 ==================