[Doc] Add an example of custom CRUD action

[alexislefebvre]

On this section: https://symfony.com/bundles/EasyAdminBundle/current/actions.html#adding-custom-actions

There is an example of code:

public function renderInvoice(AdminContext $context)
{
    $order = $context->getEntity()->getInstance();

    // add your logic here...
}

What is the expected logic? Must I return an instance of Symfony\Component\HttpFoundation\Response?

It would be nice to have a real example.

So far, returning a Response work but it isn't in the context of this admin so that looks weird.