Support translation of documents

@idebr: If you are still interested in the issue, you may provide more information on what should be taken into account.
By means of the documentation, the task of translating documents is related to "real files", which are uploaded to drupal.
I'm not sure, if this is even possible within tmgmt.

My project requirement is pretty basic:
- Upload a file in a node
- Automatically translate the file when translating the node using Deepl

This is supported by tmgmt, see ✨ Translate files / attachments Fixed

The priority for automatic translations for documents in my project has been lowered, so I don't expect to do any development on this feature in the short term

TMGMT enables additional settings when a translator plugin includes "files = TRUE" in its annotation

1. \Drupal\tmgmt_deepl\DeeplTranslatorApiInterface::translate is renamed to \Drupal\tmgmt_deepl\DeeplTranslatorApiInterface::translateText to match the deepl client method
2. Introduced \Drupal\tmgmt_deepl\DeeplTranslatorApiInterface::translateDocument

I will continue to iterate on this issue next week.

1. Translatable files are now added as a separate batch operation per file, see \Drupal\tmgmt_deepl\DeeplTranslatorBatchInterface::translateDocumentOperation
2. A document is translated by Deepl through \Drupal\tmgmt_deepl\DeeplTranslatorApiInterface::translateDocument

It would be nice, if you could add testing instructions on how to use the document translation.
I think i'll find some time next week for further testing - right now i'm heavily involved in projects at work.

TMGMT supports translation of files on a conceptual level, see https://www.drupal.org/node/3392215 →

The mimetypes that should be translated can be selected at /admin/tmgmt/settings under 'File translation'

Assuming you have a working setup with nodes, you can add a 'File upload' field to the content type and check the option 'Users may translate this field' in the field settings.

DeeplTranslatorBatch:: getTranslationOptions() is a bit messy right now since the settings priority is currently not implemented (Job settings > Translator settings -> Translator plugin defaults). I filed a follow-up issue to start smoothing this out, see 📌 Provide sensible default settings Active

I've build up a simple setup with a basic content type containing a translatable file field.
After attaching a PDF file and hitting the translation workflow the PDF is being translated without issues.
The translated file gets attached to the translated node and results look fine. DeepL is doing a good job while translating and keeps the structure/ formatting of the document.
I've attached the PDF as an example in here.

The MR will be merged into 2.3.x.

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