Add docs for contract storage layout and persistent key naming conventions

[Chucks1093]

Summary

The contract's persistent storage layout is undocumented, making it difficult for new contributors to understand what is stored, under which keys, and for how long. A reference document should describe the full storage layout and the key naming conventions used.

Scope

• List every persistent storage key used by the contract
• For each key: key format, data type stored, TTL policy, and which functions read and write it
• Document the naming convention used for composite keys (e.g. creator + holder balance keys)
• Include a note on TTL extension triggers and the minimum ledger threshold that triggers extension

Acceptance Criteria

• All persistent storage keys documented
• Each entry includes key format, data type, TTL policy, and read/write functions
• Composite key naming convention explained with an example
• TTL extension behaviour and threshold documented
• Document lives in docs/ in the contracts repo

ETA: 12 hours

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Coordinate on Telegram

[Luchistack]

Hi Maintainer, i would love to wok on this issue.

[olaayoade91-byte]

hello i would like to work on this please assign me

[Menjay7]

Hi! I came across this issue and would love to work on it. I have experience with similar tasks and I'm confident I can investigate, implement a clean solution, and thoroughly test it before submitting a PR. If the issue is still open, I would appreciate it if you could assign it to me. I’ll prioritize this immediately and ensure a reliable solution is delivered as soon as possible. Thank you!