Document the upgrade authority model

[arisu6804]

Summary

Document the upgrade authority model.

Context

Tracking issue for the StreamPay smart contract. Open to contributors.

Acceptance criteria

• Implement the change described above
• Add or update automated tests
• Update the relevant documentation

[nurudeenheroic]

I've gone through the issue and am ready to get started.

[grantfox-oss]

🦊 GrantFox — @nurudeenheroic has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #60)
2. Your PR will be reviewed by the StreamPay-Organization maintainers

Good luck! Track your progress on GrantFox.