Feat/add storage ttl doc

[AugistineCreates]

This pr closes #1577

This PR resolves the issue by introducing a central reference document for the Time-To-Live (TTL) storage keys and extension strategy used across the Soroban smart contracts. It maps out each key, its storage class, and bump parameters, preserving tribal knowledge and making reviewers and contributors more productive.

Type of Change

• Documentation update

Changes Made

Files Modified

• README.md
• quicklendx-contracts/README.md

New Files Added

• docs/STORAGE_TTL.md

Key Changes

• Documented all Persistent and Instance keys used in the codebase.
• Mapped keys to PERSISTENT_TTL_THRESHOLD (~402 days) where applicable.
• Provided concrete, compiler-ready Rust examples demonstrating how TTL extensions should be handled when reading, writing, or sweeping keys.
• Cross-linked the documentation from root and contract-level READMEs.

Testing

• Manual testing completed (Verified code formatting and validity of Rust/Soroban SDK examples)
• No breaking changes introduced (No logic modifications made)

Contract-Specific Checks

• Security considerations reviewed (Ensured all documented details match current implementation behavior)

Code Quality

• Functions/keys are properly documented
• Code formatting follows standards

Documentation

• README updated
• API/Storage documentation updated

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[Baskarayelu]

add storage ttl doc — solid — merging 👍