Form in Symfony controller

There is nothing wrong in using forms in Controller in my opinion. However if you want to keep your controllers slim you can move your business logic to services. For example:

App\Controllers\HomeController:

class HomeController extends Controller
{
    // Inject ArticleService
    public function index(Request $request, ArticleServiceInterface $articleService)
    {
        $article = new Article();
        $form = $this->createForm(ArticleType::class, $article);
        $form->handleRequest($request);

        if ($form->isSubmitted() && $form->isValid()) {

            $articleService->create($form->getData());

            return $this->redirect("/");
        }
        return $this->render('home/home.html.twig', [
            'form' => $form->createView()
        ]);
    }
}

App\Services\ArticleService:

class ArticleService implements ArticleServiceInterface
{
    private $entityManager;

    public function __construct(EntityManagerInterface $entityManager) {
        $this->entityManager = $entityManager;
    }

    public function create(Article $article)
    {
        // you may do some extra stuff here
        $this->entityManager->persist($article);
        $this->entityManager->flush();
    }
}

This is a complete overkill in this particular example, but gives an idea how it may be.

More info in Symfony docs.

In one medium-big proyect (or to be more orderly) you must put in src/Form/Type/ folder (you must create it). In your example:

<?php
// src/Form/Type/ArticleType.php
namespace App\Form\Type;

use App\Entity\Article;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolver;

//you must use the classes you use in the form
use Symfony\Component\Form\Extension\Core\Type\CollectionType;
use Symfony\Component\Form\Extension\Core\Type\CheckboxType;

class ArticleType extends AbstractType
{
   public function buildForm(FormBuilderInterface $builder, array $options)
   {
    //add your form code here
      $builder
         ->add()
         ->add ()
       ;
   }

   public function configureOptions(OptionsResolver $resolver)
   {
      $resolver->setDefaults(array(
      //write here your options
      ));
   }

   public function getBlockPrefix()
   {
      return null;
   }

}