Correct class namespacing in the user guide

[jim-parry]

A sweep through the user guide, to correct class namespacing declarations.
Fixes #1401

[jim-parry]

@lonnieezell Thanks for checking. Sorry about the typos!
I think I am clearer on the namespacing now :-/

[hwiesmann]

Hi,

I think there is another one on CodeIgniter4/tutorial/static_pages.html. A backslash is missing before CodeIgniter:
<?php namespace App\Controllers; class Pages extends CodeIgniter\Controller {

[jim-parry]

@hwiesmann I thought I got that.
The original "namespace App\Controllers; class Pages extends CodeIgniter\Controller {" should become "namespace App\Controllers; use CodeIgniter\Controller; class Pages extends Controller {" after all my changes.