Allow rendered entity as a display format

Thanks for putting this together.

I think adding some documentation for using '#type' => 'processed_text' to render a simple block would also be really useful. It might also be worth adding the necessary logic to make the entities renderable on their own.

Created a new MR.

It contains a new formatter for entity reference fields to render simple block entities.
The patch #2 is not working at all because:
- there are no dependency injections;
- it doesn't allow to render multiple values;
- file name does not start with the module name;
- etc.

We cannot also render entities on their own because the entity class doesn't have view_builder handler.

Merged the MR with the 'Rendered Simple Block' formatter.

Automatically closed - issue fixed for 2 weeks with no activity.