[Docs] Add SEP-41 compliance mapping for invoice-token

[Jayrodri088]

Complexity / points: Medium (150 points) - maintainer label: points-150-medium

PR must include:

• Closes #12 in the PR description
• Tests added or updated where behavior changes
• Docs updated if public API, lifecycle, or deploy steps change
• No unwrap() in production Soroban contract paths (project convention)
• cargo test passes locally; match CI (cargo fmt, cargo clippy -D warnings) when touching Rust

Problem

invoice-token is described as SEP-41 compliant, but the repo does not include a clear compliance mapping document.

Without explicit documentation, auditors and integrators must infer behavior from Rust source and tests, which slows review and increases misinterpretation risk.

Proposed change

Add a doc that maps this token implementation to the key SEP-41 requirements and documents any deviations.

Acceptance criteria

• Create docs/SEP41_COMPLIANCE.md (or similarly named file)
• Document how each SEP-41 relevant function maps to this contract implementation:
  • transfer, approve, transfer_from
  • allowance/expiration behavior
  • mint, burn, burn_from
• Document the project-specific behavior (transfer_locked restriction) and how it affects standard flows
• Document event publishing behavior (transfer, approve, mint, burn) and expected topic/data shapes
• Link this doc from docs/API.md or Readme.md

Dependencies

Recommended after token behavior tests are expanded (so behavior is well specified): [Testing] Expand invoice-token SEP-41 behavior tests.

[Jayrodri088]

Deferred (Wave 1 scope)
Maintainers are focusing this wave on implementation: escrow, invoice-token lifecycle, settlement/payment flows, payment-distributor, tests, and security bugs — aligned with decentralized invoice factoring on Stellar (liquidity for sellers, yield for investors, on-chain escrow/settlement).

Documentation issues will be reopened or recreated in a later wave. If you already started work, comment here and we can link a follow-up issue.