Write a Phoenix Contexts -> Ash Resources guide

[zachdaniel]

This guide can really help contextualize how to use Ash, and would be a great reference. We could start with running some phoenix generators, and then show how the same thing would be accomplished with Ash.

[chrishop]

Does this not cover the issue?
#70

[zachdaniel]

This is like a migration guide from phoenix context code (i.e say you have already implemented a phoenix app, how would you migrate it to Ash)