[Security] Update custom authenticator docs to include identifier normalization

[Spomky]

Fixes #20632

Improve the documentation for custom authenticators by detailing the concept of user identifier normalization. Introduced the NormalizedUserBadge example to demonstrate how to simplify and standardize identifiers.

[Spomky]

Many thanks.
I will take time to update this section.

[Spomky]

It should be fixed with the last commit

[alamirault]

With the other note below, I'm not sure this one is needed

[Spomky]

I believe both notes provide valuable context and would prefer to keep tthem:

• The first shows a well-known real-world example (Google).
• The second one highlights enterprise-specific formats where normalization is important.

[alamirault]

• Php code was not rendered. I added :: and .. code-block:: php (Not 100% sure it's the best way)
• I replace (string) $request->request->get() by $request->request->getString

Suggested change

	For instance, the example below uses a normalizer that converts usernames to a normalized, ASCII-only, lowercase format,
	suitable for consistent comparison and storage.
	// src/Security/NormalizedUserBadge.php
	namespace App\Security;
	use Symfony\Component\Security\Http\Authenticator\Passport\Badge\UserBadge;
	use Symfony\Component\String\UnicodeString;
	use function Symfony\Component\String\u;
	final class NormalizedUserBadge extends UserBadge
	{
	public function __construct(string $identifier)
	{
	$callback = static fn (string $identifier) => u($identifier)->normalize(UnicodeString::NFKC)->ascii()->lower()->toString();
	parent::__construct($identifier, null, $callback);
	}
	}
	// src/Security/PasswordAuthenticator.php
	namespace App\Security;
	final class PasswordAuthenticator extends AbstractLoginFormAuthenticator
	{
	// Simplified for brievety
	public function authenticate(Request $request): Passport
	{
	$username = (string) $request->request->get('username', '');
	$password = (string) $request->request->get('password', '');
	$request->getSession()
	->set(SecurityRequestAttributes::LAST_USERNAME, $username);
	return new Passport(
	new NormalizedUserBadge($username),
	new PasswordCredentials($password),
	[
	//All other useful badges
	]
	);
	}
	}
	.. note::
	For instance, the example below uses a normalizer that converts usernames to a normalized, ASCII-only, lowercase format,
	suitable for consistent comparison and storage::
	// src/Security/NormalizedUserBadge.php
	namespace App\Security;
	use Symfony\Component\Security\Http\Authenticator\Passport\Badge\UserBadge;
	use Symfony\Component\String\UnicodeString;
	use function Symfony\Component\String\u;
	final class NormalizedUserBadge extends UserBadge
	{
	public function __construct(string $identifier)
	{
	$callback = static fn (string $identifier) => u($identifier)->normalize(UnicodeString::NFKC)->ascii()->lower()->toString();
	parent::__construct($identifier, null, $callback);
	}
	}
	.. code-block:: php
	// src/Security/PasswordAuthenticator.php
	namespace App\Security;
	final class PasswordAuthenticator extends AbstractLoginFormAuthenticator
	{
	// Simplified for brievety
	public function authenticate(Request $request): Passport
	{
	$username = $request->request->getString('username');
	$password = $request->request->getString('password');
	$request->getSession()
	->set(SecurityRequestAttributes::LAST_USERNAME, $username);
	return new Passport(
	new NormalizedUserBadge($username),
	new PasswordCredentials($password),
	[
	//All other useful badges
	]
	);
	}
	}
	.. note::