Add cc-by-nc-2.0 to list of licenses
_The documentation is not available anymore as the PR was closed or merged._ this list needs to be kept in sync with the ones in moon-landing and hub-docs :) @julien-c don't you think it might be better to a have a single file (source of truth) in one of the repos and then use it in every other repo, instead of having 3 copies of the same file that must be kept in sync?

Also note that the licenses we are adding were all already present in our previous `licenses.json` file: are we regenerating it, step by step? Why don't we use a file with ALL the licenses we previously had in the list?

Licenses added:
- #4887
- #4930 

Previous `licenses.json` file:
- https://github.com/huggingface/datasets/blob/b7612754928e0fd43b9e3c3becb906ec280ff5d4/src/datasets/utils/resources/licenses.json
- removed in this commit: https://github.com/huggingface/datasets/pull/4613/commits/9f7725412dac1089b3e057f9e3fcf39cc222bc26

Let me know what you think and I can take care of this. > Let me know what you think and I can take care of this.

What I think is that we shouldn't add licenses that are just used in a couple of datasets, and just use `license_details` for this.

> don't you think it might be better to a have a single file (source of truth) in one of the repos and then use it in every other repo, instead of having 3 copies of the same file that must be kept in sync?

Yes, in my opinion we can just delete this file from `datasets`, the validation is happening hub-side anyways now? 
 Feel free to delete the license list in `datasets` @albertvillanova ;)

Also FYI in #4926 I also removed all the validation steps anyway (language, license, types etc.)
This PR adds the `cc-by-nc-2.0` to the list of licenses because it is required by `scifact` dataset: https://github.com/allenai/scifact/blob/master/LICENSE.md