Development: update steps for testing subscriptions

[stsewd]

• Mention the use of environment variables for testing
• Add a default for the stripe key (otherwise the instance won't start)

Hit this today while spinning up my dev environment

Ref #10959

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📚 Documentation previews 📚

• User's documentation (docs): https://docs--10992.org.readthedocs.build/en/10992/

• Developer's documentation (dev): https://dev--10992.org.readthedocs.build/en/10992/

[agjohnson]

This was somewhere on my list, thanks for taking it off! Next step, I'd like to ensure we're all using Stripe keys when we run our instances, instead of relying on Stripe not working locally.