Add DocumentNDCGEvaluator component

[julian-risch]

Some datasets contain document relevance scores or at least a sorted order of ground truth relevant documents, for example HotpotQA. In order to evaluate rankers and retrievers regarding their ability to not only distinguish relevant from non-relevant documents but also to sort relevant documents by their relevance, we need a new component DocumentNDCGEvaluator. The currently available DocumentMRREvaluator considers relevancy of documents to be binary.

[srini047]

@julian-risch I would like to contribute to this new evaluator.

[julian-risch]

Hello @srini047 thanks for the initiative and for reaching out. Unfortunately it's too late this time as we already started working on it, sorry. I hope you find another issue in our contributions wanted list that you would like to contribute to. Thank you! 🙏