docs: add initial configuration

It is required.

Author: kenjis

user_guide_src/source/installation/installing_composer.rst

@@ -52,7 +52,11 @@ A sample such installation command, using the default project-root "appstarter":
 
     > composer create-project codeigniter4/appstarter --no-dev
 
-After installation you should follow the steps in the "Upgrading" section.
+Initial Configuration
+---------------------
+
+After installation, a few initial configuration is required.
+See :ref:`initial-configuration` for the detail.
 
 .. _app-starter-upgrading:
 
@@ -148,6 +152,12 @@ Copy the ``env``, ``phpunit.xml.dist`` and ``spark`` files, from
 You will have to adjust the system path to refer to the vendor one, e.g., ``ROOTPATH . '/vendor/codeigniter4/framework/system'``,
 - the ``$systemDirectory`` variable in **app/Config/Paths.php**
 
+Initial Configuration
+---------------------
+
+A few initial configuration is required.
+See :ref:`initial-configuration` for the detail.
+
 .. _adding-codeigniter4-upgrading:
 
 Upgrading

user_guide_src/source/installation/installing_manual.rst

@@ -26,10 +26,11 @@ and extract it to become your project root.
     The symbols that can be used are ``/``, ``_``, ``.``, ``:``, ``\`` and space.
     So if you install CodeIgniter under the folder that contains the special characters like ``(``, ``)``, etc., CodeIgniter won't work.
 
-Setting Up
+Initial Configuration
 =====================
 
-None
+After installation, a few initial configuration is required.
+See :ref:`initial-configuration` for the detail.
 
 .. _installing-manual-upgrading:
 

user_guide_src/source/installation/running.rst

@@ -12,6 +12,8 @@ This section addresses how to use each technique, and explains some of the pros
 If you're new to CodeIgniter, please read the :doc:`Getting Started </intro/index>`
 section of the User Guide to begin learning how to build dynamic PHP applications. Enjoy!
 
+.. _initial-configuration:
+
 Initial Configuration
 =====================