MeDAL dataset: Add further description and update download URL
@lhoestq I'm a bit confused by the error message. I haven't touched the YAML code at all - do you have any insight on that? I just added the missing `pretty_name` tag in the YAML - sorry about that ;) Thanks! Seems like it did the trick since the tests are passing. Let me know if there's anything else I can do in this PR! It's all good thank you :)

merging !
Added more details in the following sections:
* Dataset Structure
* Data Instances
* Data Splits
* Source Data
* Annotations
* Discussions of Biases
* LIcensing Information