Contracts docs: `group_treasury` storage layout

[codebestia]

Write contracts/docs/contracts-group-treasury-storage.md documenting contracts/contracts/group_treasury/src/storage.rs and token_interface.rs — member list storage, per-token balance storage, and the withdrawal-proposal record structure (fields, status enum values).

Acceptance criteria:

• Every storage key/data structure is documented with its type and what it represents
• The WithdrawProposal struct's fields and status transitions are documented as a small state diagram
• Storage type (instance/persistent/temporary) and any TTL/bump behavior is documented

[abdulquadri123]

@abdulquadri123 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hey Maintainer,
As a blockchain developer, I will delighted to handle this issue and solve it diligently once assigned to me.
Thanks.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @abdulquadri123 to this issue.

[drips-wave]

Congratulations, @abdulquadri123! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on July 30, 2026.

🧑‍💻 @abdulquadri123: 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.

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