Add "Model Explanation" section to the User Guide

[Andrew DalPino]

With the addition of Decision Tree Graphviz visualizations and given that some Learners already implement the RanksFeatures interface which provides a method to output the importance scores of each feature in the training set, we could start to build out a separate section of the User Guide dedicated to model explainability.

I think a good place to start would be an Introduction, a Feature Importances section, and a Decision Tree visualization section. We could move the Feature Importances section over from the Training page (https://github.com/RubixML/ML/blob/master/docs/training.md#feature-importances). We should also include an image (png) of an example Decision Tree graph.

The page should be written in markdown like the rest of them see https://github.com/RubixML/ML/tree/master/docs.