add a note about the env vars needed for email in quick-start

[jdoconnor]

• Mirror the "remember to update ..." for setting up email sending in the quick-start.
• make the "remember to update ..." alerts/callouts/admonitions (interchangable word usage)

reference: https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#alerts