Problem

Every core module has an API reference page under docs/api/ (hot_reload.md, diagnostics.md, screen.md, and so on), but src/pythonnative/preview.py has none. The desktop preview only has the guide at docs/guides/desktop-preview.md, so the public run_preview() (line 371) and DesktopApp (line 76) have no rendered reference docs.

Suggested fix

• Add docs/api/preview.md using the same thin mkdocstrings pattern as an existing page (for example, docs/api/hot_reload.md): a short intro plus a ::: pythonnative.preview block.
• Add the page to the API section of the nav in mkdocs.yml (around lines 105-135), keeping alphabetical order with its neighbors.
• Link the new page from docs/guides/desktop-preview.md where it discusses run_preview.

Verification

Run mkdocs build --strict and confirm the page renders with mkdocs serve.

Suggested commit message

docs(mkdocs): add preview API reference page