Document special steps for deploying Open Saas

[sodic]

During the 7/18 incident (when someone deleted all our Fly apps), we realized that most of the example apps we care about require special steps (e.g., Mage, wasp-bot). This also includes Open Saas.

One example is DNS. The best way to collect all the stuff to document is deploying an Open Saas replica from scratch and writing stuff down.

Consider implementing these first. They aren't necessarily blockers but will likely mimize the work needed for this task:

• https://github.com/wasp-lang/internal/issues/69
• Save app production env vars to dotenv vault wasp#3435
• Document a clear general procedure for reviving applications wasp#3418
• Improve Wasp's deployment artifacts wasp#3417