add dataset contributors for acknowledgements

[pbellec]

We now have very neat contributor lists for each dataset, e.g. https://github.com/courtois-neuromod/hcptrt/

we should include some recommendations in https://docs.cneuromod.ca/en/latest/ACKNOWLEDGMENT.html for people to acknowledge the contributors of the dataset they are using.

[pbellec]

actually, it's better to point to the website for up-to-date information. Listing all contributors will lead to bloated acknowledgment sections, especially for papers combining multiple cneuromod datasets.