docs: adds code push quickstart guide

[bryanoltman]

Description

Adds our first tutorial, which is the shortest possible zero-to-apply-patch guide. The goal is to give potential users a quick route to the "ah ha" moment for code push.

Followups

• docs: add glossary of terms #35
• Write down instructions that would result in TTs having done what we want them to do (ship an app). #25