[Docs] Add per-contract README usage guides

[Jayrodri088]

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

PR must include:

• Closes #16 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

The repo has one root Readme.md, but there are no contract-specific README files under contracts/invoice-escrow, contracts/invoice-token, or contracts/payment-distributor.

That makes it harder for contributors/integrators to:

• find the contractΓò¼├┤Γö£ΓòóΓö¼├║╬ô├╢┬ú╬ô├╢├⌐╬ô├▓┬╝Γö£Γöñ╬ô├╢┬ú╬ô├«├ë╬ô├╢┬╝╬ô├▓┬Ñ╬ô├▓┬╝Γö£Γöñ╬ô├╢┬úΓö£ΓûÆ╬ô├╢┬ú╬ô├╢├⌐s purpose
• find example call flows
• understand each contractΓò¼├┤Γö£ΓòóΓö¼├║╬ô├╢┬ú╬ô├╢├⌐╬ô├▓┬╝Γö£Γöñ╬ô├╢┬ú╬ô├«├ë╬ô├╢┬╝╬ô├▓┬Ñ╬ô├▓┬╝Γö£Γöñ╬ô├╢┬úΓö£ΓûÆ╬ô├╢┬ú╬ô├╢├⌐s inputs, auth expectations, and error cases

Proposed change

Add per-contract README files with a small, consistent structure.

Acceptance criteria

• Create contracts/invoice-escrow/README.md
• Create contracts/invoice-token/README.md
• Create contracts/payment-distributor/README.md (documenting current stub status if still unimplemented)
• Each README includes:
  • purpose + high-level flow
  • how to initialize/configure the contract
  • example call sequence (aligned with Soroban CLI commands in root README)
  • link to docs/API.md (once created)
• README examples do not include real secrets (use placeholders)

Dependencies

Recommended after [Docs] Add missing docs/API.md for contract public methods so the per-contract docs can link to a single source of truth.

[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.