Sway Ref: Style Guide single use variables

[Braqzen]

Description

Closes #4661

Checklist

• [x] I have linked to any relevant issues.
• [ ] I have commented my code, particularly in hard-to-understand areas.
• [x] 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.
• [x] I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• [x] I have requested a review from the relevant team or maintainers.

[K1-R1]

How did I double approve 🤔