Adding Python notebook for native function examples (microsoft#699)

### Motivation and Context
This PR is intended to give some functioning examples to help users get
started with python native functions.
Currently, there are no examples or very limited examples. When I was
playing around with this tool I ended up using some of the integration
tests as examples to help get started. Hopefully, this will help the
next user with the lessons learned.


### Description
This adds a jupyter notebook that walks users through using native
functions alongside semantic functions to build a simple story writing
app. This notebook also shows how to use ContextVariables and run a
semantic function from within a native function.

Co-authored-by: Carter Williamson <carter.williamson@anitian.com>
Co-authored-by: Devis Lucato <devis@microsoft.com>
Co-authored-by: Alex Chao <5111035+alexchaomander@users.noreply.github.com>