Create a list of use cases

[jdebacker]

ParamTools docs should have a clear list of use cases (e.g., Tax-Brain, Tax-Calculator, Cost-of-Capital-Calculator, OG-USA). These not only highlight what ParamTools has done, but might provide nice examples for others to build off of.

[hdoupe]

+1. Thanks for the idea @jdebacker. I'll add this to the documentation.