Document the initial public API and scope #2

Provide a guide for generating public API Markdown and running lint without Rigor. Rigor lens integration and formatting follow 0.0.1.

Acceptance criteria

• Show installation, parse/render APIs, and CLI directory/stdin input.
• Explain documentation markers, multiline descriptions, main tags, and contract/API documentation separation.
• Explain the versioned model, diagnostics, SourceSpan, and API compatibility.
• Document exit codes, --fail-on warning, and rendering errors without partial Markdown.
• State supported Ruby versions, UTF-8 requirements, visibility rules, and static analysis limits; runtime name resolution and Rigor claim checking are outside this release.
• Run examples and confirm their documented output.

Dependencies: none.

Status

Completed in #7 after Sol approval and CI.