Document training.json config files

[DannyWeitekamp]

Need documentation on what is allowed in training files including:
-Overview of the different levels of attributes: global, training_et, agent, problem_set
-What attributes are legal at each level
-What attributes are open-ended (for example depends on agent or outerloop)
-Documentation of eval functions

[eharpste]

We do have this: https://github.com/apprenticelearner/AL_Train/wiki/Training.json-Specifications which we probably need to update and want to migrate over to readthedocs.

Also, do we want to consider using some kind of json schema validator that could throw intelligent errors when given bad configurations?