Document process to add new environment variables

[tim-schilling]

I'm unsure of how to add new environment variables for our staging and production environments. This blocks me from being able to address other issues such as #114. I suspect the process already exists, it just need to learn what it is.

[dawnwages]

Here's a video!
https://drive.google.com/file/d/1pUXEZO-msJWPgKyM8pQF5RIcFEAUBklO/view?usp=drive_link

Here's longer documentation: https://docs.google.com/document/d/1T47kDAkVUQ_yC89y9RZ049KRskvNiObA_YGiIsnBWeY/edit?usp=sharing

Let me know if there's anything else you need.

[tim-schilling]

Thanks @dawnwages. Is the following a good summary?

1. Add new repository secret to our GitHub repo
2. Add environment variable to the relevant deployment GitHub action workflows
3. Trigger build for the correct platform via GitHub action