Documentation for deploying app versions

[afwillia]

Is your feature request related to a problem? Please describe.
Add documentation, like a slide deck, for how to deploy the main, staging, and testing instances of the app.

Describe the solution you'd like
A clear and concise description of what you want to happen.

How important is this feature? Select from the options below:
• 🌗 Medium - can do work without it; but it's important (e.g. to save time or for convenience)

When will use cases depending on this become relevant? Select from the options below:
• Short-term - 2-4 weeks

[milen-sage]

Documentation for shinyapps.io

[afwillia]

Related to #280