add equations to API docs

[knaaptime]

sometimes the same index carries different names in different fields (e.g. Herfindahl in econ versus Simpson's Diversity in biology), so we need to ensure the equation for each index is exposed in the documentation). The best way to do that is probably to include the equation in latex in the Notes section of each index's docstring, which should render it on on the API docs page (though open to other suggestions)

[knaaptime]

it would also be great if the latex equation string was available as an attribute of each class