docs: add user guide

Author: kenjis

user_guide_src/source/changelogs/v4.3.0.rst

@@ -52,6 +52,7 @@ Changes
     - The ``CodeIgniter\CLI\CommandRunner`` class has been removed due to a change in Spark commands processing.
     - The system route configuration file ``system/Config/Routes.php`` has been removed.
     - The route configuration file ``app/Config/Routes.php`` has been changed. Removed include of system routes configuration file.
+- All union type properties in ``Config`` classes have been typed.
 
 Deprecations
 ************

user_guide_src/source/installation/upgrade_430.rst

@@ -30,6 +30,17 @@ The following files received significant changes and
         > composer update
         > cp vendor/codeigniter4/framework/spark .
 
+Config Files
+============
+
+If you are using the following Mock Config classes in testing, you need to update the corresponding Config files in **app/Config**:
+
+- ``MockAppConfig`` (``Config\App``)
+- ``MockCLIConfig`` (``Config\App``)
+- ``MockSecurityConfig`` (``Config\Security``)
+
+Add **types** to the properties in these Config classes.
+
 Breaking Changes
 ****************