fix: change package cdn from unpkg to jsdelivr

[DigTheDoug]

Summary

Running this sandbox does not currently work as the @mui/material package is not found. It does not look like unpkg is actively supported any longer, so switching to jsdelivr instead

Checklist

• [x] Follows Contributing guidelines.
• [x] Pull Request title uses Conventional Commit syntax.
• [x] All tests pass.
• [x] Linted code.
• [x] Authored new tests, if necessary.
• [x] Updated documentation, if necessary.