docs: add upgrading guide for v4.2.12

Author: kenjis

user_guide_src/source/installation/upgrade_4212.rst

@@ -0,0 +1,27 @@
+###############################
+Upgrading from 4.2.11 to 4.2.12
+###############################
+
+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
+
+Project Files
+*************
+
+Version ``4.2.12`` did not alter any executable code in project files.
+
+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:
+
+* app/Controllers/BaseController.php
+* composer.json

user_guide_src/source/installation/upgrading.rst

@@ -16,6 +16,7 @@ See also :doc:`./backward_compatibility_notes`.
 
     backward_compatibility_notes
 
+    upgrade_4212
     upgrade_4211
     upgrade_4210
     upgrade_428