docs: add user guide

Author: kenjis

user_guide_src/source/changelogs/v4.5.0.rst

@@ -451,10 +451,22 @@ Changes
 Deprecations
 ************
 
-- **CodeIgniter:** The ``determinePath()`` method has been deprecated. No longer
-  used.
 - **Services:** The ``BaseService::$services`` property has been deprecated. No
   longer used.
+- **CodeIgniter:**
+    - The ``determinePath()`` method has been deprecated. No longer used.
+    - The ``resolvePlatformExtensions()`` method has been deprecated. No longer
+      used. It has been moved to the ``CodeIgniter\Boot::checkMissingExtensions()``
+      method.
+    - The ``bootstrapEnvironment()`` method has been deprecated. No longer used.
+      It has been moved to the ``CodeIgniter\Boot::loadEnvironmentBootstrap()``
+      method.
+    - The ``initializeKint()`` method has been deprecated. No longer used. It has
+      been moved to the ``Autoloader``.
+    - The ``autoloadKint()`` method has been deprecated. No longer used. It has
+      been moved to the ``Autoloader``.
+    - The ``configureKint()`` method has been deprecated. No longer used. It has
+      been moved to the ``Autoloader``.
 - **Response:** The constructor parameter ``$config`` has been deprecated. No
   longer used.
 - **Filters:**

user_guide_src/source/installation/upgrade_450.rst

@@ -15,6 +15,26 @@ Please refer to the upgrade instructions corresponding to your installation meth
 Mandatory File Changes
 **********************
 
+index.php and spark
+===================
+
+The following files received significant changes and
+**you must merge the updated versions** with your application:
+
+- ``public/index.php``
+- ``spark``
+
+.. important:: If you don't update the above files, 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
+        cp vendor/codeigniter4/framework/spark spark
+
 Breaking Changes
 ****************