Fix the binary being broken with Symfony Console 8

[Amoifr]

As mentioned in #208: bin/docs-builder currently dies on startup with Symfony Console 8.

$ php bin/docs-builder list
PHP Fatal error: Call to undefined method Symfony\Component\Console\Application::add() in src/Application.php:38

Application::add() was deprecated in Symfony 7.4 in favor of addCommand(), and removed in 8.0. Since composer.json allows symfony/console: ^5.2 || ^6.0 || ^7.0 || ^8.0, the call is guarded rather than swapped outright — addCommand() only exists from 7.4 on:

if (method_exists($this->application, 'addCommand')) {
    $this->application->addCommand($command);
} else {
    $this->application->add($command);
}

Why CI never caught this: the whole suite drives the command through CommandTester, so the binary — and therefore Application — is never executed. The new test runs bin/docs-builder list as a subprocess and asserts it boots and registers build:docs. It fails without the fix (exit code 255, the fatal above) and passes with it.

Verified by hand too: a full bin/docs-builder build:docs <source> <output> --symfony-version=4.0 now completes and writes the HTML files.

Full suite green: 69 tests, 156 assertions.