Only run docs CI checks when docs have changed

[sdankel]

Description

This makes it so the Docs CI checks only happen when a PR includes changes to files under docs/

Checklist

• [ ] I have linked to any relevant issues.
• [ ] I have commented my code, particularly in hard-to-understand areas.
• [ ] I have updated the documentation where relevant (API docs, the reference, and the Sway book).
• [ ] I have added tests that prove my fix is effective or that my feature works.
• [ ] I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• [ ] I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• [ ] I have requested a review from the relevant team or maintainers.