What

• Add pn codegen to the CLI subcommand reference.
• Document --output, its default directory, and repeatable --module imports.
• Add an explicit output-directory example and links to the native-contracts and generated-source documentation.

Why

pn codegen is available in the CLI but was missing from the hand-written subcommand reference.

How

Document the current pn codegen behavior and clarify that it generates contract files and prints their paths, while native compilation and app rebuilding happen in the build workflow.

Testing

• Verified uv run pn codegen --help.
• Ran uv run pn codegen --output /tmp/pythonnative-contracts and confirmed the generated files and printed paths.
• Ran uv run --group docs mkdocs build --strict successfully.

Risks/Impact

Documentation-only change. No runtime behavior is modified.

Docs/Follow-ups

No follow-up changes required.

Closes #96