docs: change the first sentence

From https://github.com/codeigniter4/CodeIgniter4?tab=readme-ov-file#what-is-codeigniter

Author: kenjis

user_guide_src/source/intro/index.rst

@@ -2,7 +2,10 @@
 Welcome to CodeIgniter4
 #######################
 
-CodeIgniter is an Application Development Framework - a toolkit - for
+CodeIgniter is a PHP full-stack web framework that is light, fast, flexible and
+secure.
+
+It is an Application Development Framework - a toolkit - for
 people who build web sites using PHP. Its goal is to enable you to
 develop projects much faster than you could if you were writing code
 from scratch, by providing a rich set of libraries for commonly needed