Document each helpafamily env var in its own page in env_vars/

[audreyfeldroy]

For each environment variable (or group of env vars, as with the Contentful ones) used in the helpafamily project:

• Create a new .md file in env_vars/
• Write brief instructions about how to create the environment variable
• Link to any relevant third-party documentation on how to create it, if any exists
• Add screenshots if anything feels confusing to newbies

Kindly submit 1 PR per env var grouping, to make your PRs easy to review 🙂 Post a comment with what env vars you're documenting before you start.