Update docs for new class imbalance support

[ablaom]

After

• Include MLJBalancing.jl in MLJ and re-export it's names.  #1062

we need to do:

• Include the new models provided by Imbalance.jl and the model EnsembleBaggingClassifier provided by MLJBalancing into the Model Browser. We will need a new category of models, "Class Imbalance"

• Add a new manual page "Correcting for Class Imbalance " which emphasises the BalancedModel wrapper, but which can more-or-less point to the documentation for those packages.

cc @EssamWisam