docs: add external code doc

[sarahschwartz]

Description

This PR adds a page for external code execution in the Sway book.

Checklist

• [x] I have linked to any relevant issues.
• [x] 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).
  • [x] If my change requires substantial documentation changes, I have requested support from the DevRel team
• [x] I have added tests that prove my fix is effective or that my feature works.
• [x] 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.