docs: add "Creating Custom Helpers" and sample code

Author: kenjis

user_guide_src/source/general/helpers.rst

@@ -172,9 +172,17 @@ URI to the controller/method you wish to link to.
 Creating Helpers
 ****************
 
+Creating Custom Helpers
+=======================
+
 The helper filename is the **helper name** and **_helper.php**.
-For example, to create Email Helper, you’ll create a file named
-**app/Helpers/email_helper.php**.
+
+For example, to create info helper, you’ll create a file named
+**app/Helpers/info_helper.php**, and add a function to the file:
+
+.. literalinclude:: helpers/008.php
+
+You can add as many functions as you like to a single helper file.
 
 "Extending" Helpers
 ===================

user_guide_src/source/general/helpers/008.php

@@ -0,0 +1,11 @@
+<?php
+
+// app/Helpers/info_helper.php
+
+/**
+ * Returns CodeIgniter's version.
+ */
+function ci_version(): string
+{
+    return CodeIgniter::CI_VERSION;
+}