Update drug descriptor schema to add descriptor type

[sgosline]

Currently we only add the drug descriptor name and value. To date we have: SMILES, 2D_mordred, morgan, we need to add 3D mordred and document in a separate column to return one of the four.

[sgosline]

There are too many to add to the table itself or the YAML file. We will instead link out.

Update: add in link in documentation site AND link in YAML file to source of drug descriptors.