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.

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[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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[francisprecious061]

@francisprecious061 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

hello maintainer, i'd love to work on this issue! please assign me

ℹ️ Repo Maintainers: To accept this application, review their application or assign @francisprecious061 to this issue.

[Confidence001Jewel-cmd]

@Confidence001Jewel-cmd has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Dear Maintainer,
Please assign this issue to me, I will love to work on it.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Confidence001Jewel-cmd to this issue.

[Leetmanixy]

@Leetmanixy has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

I would like to work on this issue because clear documentation of smart-contract storage is important for contributors who need to understand how contract state is structured and managed. Storage keys, withdrawal proposals, status transitions, and TTL behavior can be difficult to understand by reading the implementation alone, so documenting them would make the contract easier to work with and review.

I am particularly interested in this issue because it provides an opportunity to learn the "group_treasury" contract at a deeper level while producing documentation that will be useful to future contributors. I would trace the storage definitions in "storage.rs" and the relevant interfaces in "token_interface.rs", documenting each storage key and data structure, its type, purpose, and storage lifetime.

I have experience working with software projects, technical documentation, APIs, backend systems, and debugging existing codebases. I am comfortable reading implementations and translating technical behavior into concise documentation that other developers can use.

This issue would also allow me to strengthen my understanding of Soroban smart-contract storage, including instance, persistent, and temporary storage, TTL and bump behavior, token balances, and proposal state management. I would make sure the "WithdrawProposal" fields and status transitions are accurately represented with a small state diagram and that every documented behavior matches the actual contract implementation.

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