`unpaid` subscription status undocumented

Discussed in https://github.com/Glench/ExtPay/discussions/221

^{Originally posted by ba32107 July 7, 2024}
In dev mode, I can set an unpaid status:

This status is not present in the main README or this doc:
https://github.com/Glench/ExtPay/blob/main/docs/how_subscriptions_work.md

It's not really clear what the difference is between past_due and unpaid. The Stripe docs say:

If subscription collection_method=send_invoice it becomes past_due when its invoice is not paid by the due date, and canceled or unpaid if it is still not paid by an additional deadline after that.

From this I can sort of understand the difference, but it's never mentioned anywhere in the ExtPay docs that the subscriptionStatus property mimicks the Stripe API's values. So I think this additional value should be documented in this repo.

Thanks!

Read the subscription status documentation in the main README and docs/how_subscriptions_work.md, then compare it with the Stripe subscription status documentation linked in the issue. Document the unpaid status and clarify how it differs from past_due, including that subscriptionStatus uses Stripe API values. Done means both ExtPay documentation locations accurately describe the status values.