Document Classification Node with Transformers instead of FARM

[julian-risch]

Document Classification was so far only possible with the help of FARM: https://haystack.deepset.ai/components/v0.10.0/classifier
As #1492 removes the TextClassificationHead and #1507 might also remove the TextClassificationProcessor soon, we should replace the FARM-based document classification node with a transformers-based document classification node.
#1492 removed the FARM-based document classification node, including the documentation page 5590cba
We can reuse most parts of that documentation after implementing a transformers-based document classification node.