fix: Allow multi-line descriptions to be rendered in Livebook without error

[sevenseacat]

The mix ash.generate_livebook task was generating bad Markdown for multiline attributes - the new lines were being included as-is which broke the tables.

This introduces a little helper to use breaks instead of newlines, and renders much better.

Before:

After:

Contributor checklist

• [x] Bug fixes include regression tests
• [ ] Chores
• [ ] Documentation changes
• [ ] Features include unit/acceptance tests
• [ ] Refactoring
• [ ] Update dependencies