docs: Generate API reference documentation from contract trait definitions

[nanaf6203-bit]

Description

Contract APIs are defined in traits but there is no auto-generated API reference. Use rustdoc to generate comprehensive API docs.

Acceptance Criteria

• Configure rustdoc with document-private-items for trait modules
• Add doc comments to all public trait methods and types
• Generate HTML docs via cargo doc --no-deps --document-private-items
• Add GitHub Actions workflow to publish docs to GitHub Pages
• Link generated docs from README.md
• Include cross-contract interaction diagrams in docs
• Document error types and their meanings for each trait method