feat: check for a gitignore file

[w3lld1]

Summary

• add PY008 to check that a project has a root .gitignore file
• cover present, missing, and directory-instead-of-file cases
• connect the new check to the packaging guide and regenerate the README check list

Validation

• uv run pytest -q — 257 passed, 1 skipped (the existing rate-limit-sensitive package test)
• uvx nox -s rr_lint
• uvx nox -s rr_pylint — 10.00/10
• uvx nox -s readme
• NODE_OPTIONS='--no-deprecation' npx --yes mystmd@^1.0 build --html
• git diff --check

The exact bun run build wrapper was not run locally because Bun is unavailable in this environment; the MyST HTML build itself completed successfully via npx.

Fixes #822

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📚 Documentation preview 📚: https://scientific-python-cookie--827.org.readthedocs.build/

[henryiii]

This is a good idea, thanks! Checking for a file is free in the Wasm version (it reads the file list from the API up front), so this isn't even an extra request.