Add docs on developing wrapper functions

[pzivich]

Is your feature request related to a problem? Please describe.
Documentations could be improved by addition of details on writing wrapper functions. I do this myself, but would be helpful for others who want to use delicatessen but without having to always interact with the estimating equations themselves.

Describe the solution you'd like
A new page on RTD that provides examples and advice

Describe alternatives you've considered
None

Additional context
None