docs: add a focused guide for optional output-format dependencies #346

Goal

Make it obvious how a consumer installs and diagnoses optional YAML or other output-format dependencies.

Background

Output-format support spans the renderer, optional extras, dependency errors, and the installed-package boundary. A first contributor should be able to improve this guidance without changing runtime behavior.

Scope

• Add a small table mapping each optional format to its install extra and expected diagnostic.
• Show how to choose JSON when an optional human format is unavailable.
• Link to the public output-format exception and process-boundary documentation.
• Keep the guidance valid for both installed consumers and source checkouts.

Acceptance criteria

• The table matches the current package extras and public error text.
• The fallback path is explicit and safe.
• The guide distinguishes an unavailable optional format from an unexpected internal error.
• The page is linked from the README or documentation index.
• The task can be completed with documentation and focused link checks only.

Validation

• Inspect pyproject extras and the renderer's current public message.
• Run git diff --check.
• Verify internal links.

Non-goals

• Do not change package metadata or error classification.
• Do not add dependency installation to the core runtime.
• Do not promise formats that the repository does not support.