Improve Doc-UFCN Huggingface Demo app to output JSON

We need to improve the web app to output a JSON file with enough details so that an HTR can use it to decode the transcription.

We need a new JSON Output for the web app with a dict with this format:

[
   {
       "polygon": ..., # List of the coordinates of the points
       "confidence": ..., # Confidence in the prediction for this polygon
       "channel": ..., # channel predicted for this polygon
   }, ...
]