text: Title: update docs to reflect new YB datasets module
  Body: Update the documentation to leverage new yellowbrick.datasets & create documentation on metadata on example datasets such as number of samples, number of features, target/class.
  For example, [sklearn.datasets.load_digits](http://scikit-learn.org/stable/modules/generated/sklearn.datasets.load_digits.html).
  
  Note: reminder to also update 
   - [x] the [corpus loading docs](https://www.scikit-yb.org/en/develop/api/text/corpus.html)
   - [x] contributor guide with use of datasets/fixtures in tests (both random and real)