Improve bundle templates

[davidhoelzel]

It would be nice to provide theming for this bundle, similar to the form-themes provided by symfony/twig-bridge (https://github.com/symfony/symfony/tree/7.1/src/Symfony/Bridge/Twig/Resources/views/Form).
Similar to that form-feature, we should provide blocks so that it is possible to create own themes. As a first built-in theme, we should try to implement a twbs-theme as also done in symfony-forms (for boostrap 5).

Besides theming, we should overhaul all templates to provide a better dev-experience.