Concepts docs: Group treasury multisig/authorization model

[codebestia]

Write contracts/docs/concepts-treasury-multisig-model.md explaining the group_treasury contract's authorization design: membership management, the approval-threshold model, and why withdrawal requires proposal + N approvals rather than any single member's signature.

Acceptance criteria:

• Explains the threshold model in plain terms (what "threshold" means, how it's set, whether/how it can change)
• Explains the security property this achieves and any known limitations (e.g. what happens if approvals never reach threshold, member removal mid-vote)
• Cross-references the API doc (api-group-treasury.md) for exact function signatures

[Mhidesav]

@Mhidesav has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hi
I would like to work on this issue please

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Mhidesav to this issue.

[yunus-dev-codecrafter]

@yunus-dev-codecrafter has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hi! I've carefully reviewed the entire task description and fully understand the requirements and expected outcome.
I have experience working on similar tasks and I'm confident I can complete this within about an hour. I'd be happy to take ownership of this issue and submit a clean, well-tested solution promptly.
If it's available, please assign it to me. Thank you!

ℹ️ Repo Maintainers: To accept this application, review their application or assign @yunus-dev-codecrafter to this issue.

[cythecode]

@cythecode has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hello maintainer, i’ve got this covered! Ready to drop a high-impact, clean PR that gets this merged seamlessly.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @cythecode to this issue.

[drips-wave]

Congratulations, @cythecode! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on July 30, 2026.

🧑‍💻 @cythecode: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊