docs: add and use auth()->getProvider()

[kenjis]

If a dev customizes the UserModel and changes the model name, the sample code dose not work.

[datamweb]

If a dev customizes the UserModel and changes the model name, the sample code dose not work.

Command php spark shield:model selects the name UserModel by default, if the user uses this command to generate the model, the sample codes should work.

Otherwise, it should apply the following changes.

php spark shield:model KenjiModel

-public string $userProvider = UserModel::class;
+public string $userProvider = \App\Models\KenjiModel::class;

-$users = model('UserModel');
+$users = model('KenjiModel');
$user = $users->findById(1);
$user->fill([
'first_name' => 'kenji', 
'username' =>'kenjis'
]);
$users->save($user, true);

C:\Users\ppars\OneDrive\Desktop\hi>php spark db:table users

CodeIgniter v4.3.2 Command Line Tool - Server Time: 2023-03-25 11:31:53 UTC+00:00

Data of Table "users":

+----+----------+--------+----------------+--------+--------------------+--------------------+--------------------+------------+------------+
| id | username | status | status_message | active | last_active        | created_at         | updated_at         | deleted_at | first_name |
+----+----------+--------+----------------+--------+--------------------+--------------------+--------------------+------------+------------+
| 1  | kenjis   |        |                | 1      | 2023-03-21 16:3... | 2023-03-21 15:3... | 2023-03-25 11:3... |            | kenji      |
+----+----------+--------+----------------+--------+--------------------+--------------------+--------------------+------------+------------+

In any case, using auth()->getProvider() seems good.