docs: Auto-generated contract function reference

[joelpeace48-cell]

Background & current state

Contract docs exist (docs/CONTRACTS_API.md, docs/contract-api), but a per-function reference (params, errors, events) isn't auto-generated, so it can drift from the contracts.

Goal

Auto-generate a per-function contract reference (params, returns, errors, events) from the contract spec/ABI in CI, keeping docs/CONTRACTS_API.md current.

Technical design

• Generate from the Soroban contract spec/ABI (via stellar contract spec or bindings) into markdown; CI regenerates + drift-checks (git diff --exit-code).

Task breakdown

• Generator from contract spec → markdown reference.
• CI regen + drift check.
• Cross-link from README/docs.

Acceptance criteria

• Reference regenerates on contract change; drift fails CI.

Verification

• CI regen + drift test (modify a contract → docs must update).

Difficulty: easy–medium · Effort: S–M · documentation + ci