Add documentation explaining phylogenetic beta diversity

[cjgrady]

Is your feature request related to a problem? Please describe.
We need to have documentation about phylo beta diversity.

Describe the solution you'd like

• The inline documentation should be clear as to what is going on.
• The doc strings should be updated to clearly describe the inputs and outputs of the methods.
• References should be added to the sphinx docs.
• A description of the method and how to use it should be added to the sphinx docs.

Additional context
Documentation should mirror the documentation provided for ancestral reconstruction.