Check urls of datasets in code and documentation

[ynop]

The urls used in the code and documentation should be checked for availability (As done for the readme in #87)

Places to check:

• URLs in the CorpusDownloader classes
• URLs in the code-doc of CorpusReader classes

[SuperKogito]

I think it makes sense to automate this using built-in Sphinx functionalities like the ones mentioned here.

[ynop]

I agree, it definitely should be automated. But i am not sure how, since some of the links are in the code, so the sphinx solution might not be the best. Maybe a script could check all important links and generate a small report (via shippable, github actions, ...)

[SuperKogito]

I will test the Sphinx option this weekend, if it covers the code then that will be sufficient. If not I will look into writing a script that collects all URLs and checks them.