Documentation update to include short description of data sets

[ljhwang]

The documentation includes links to available data -Accessing Available Datasets. This could be improved by adding a short description of what these datasets are.

See: https://rayleigh-documentation.readthedocs.io/en/latest/doc/source/accessing_and_sharing_data.html

[feathern]

Indeed. This got lost in the mix when we were trying to overcome the build issues with the documentation that set in near the end of the workshop. I'll try to add some descriptive text soon.