Field group doesn't work with custom entity

After deeper testing I think the issue is related to how the entity theming is being managed.

There are other issues related like 💬 Manage form display issue Active and ✨ The order of output of fields before voting Active , I also had an issue using field group module.

I will start a merge request with an approach than can fix all of them.

For anyone else trying to get field groups to show up on a custom entity display, I found the following. Working within field_layout regions too.

1. Add to theme registry.
```
/**
* Implements hook_theme().
*/
function MY_MODULE_theme($existing, $type, $theme, $path) {
return [
'MY_ENTITY' => [
'render element' => 'elements',
],
];
}
```

2. Prepare the entity template variables beneath the 'content' key.
```
function template_preprocess_MY_ENTITY(&$variables) {
$variables['view_mode'] = $variables['elements']['#view_mode'];

// Helpful $content variable for templates.
$variables += ['content' => []];
foreach (Element::children($variables['elements']) as $key) {
$variables['content'][$key] = $variables['elements'][$key];
}

}

```

3. Add MY-ENTITY.html.twig file to templates
With at least:
```
{{ content }}
```