Documentation comments for UnitExt on num

[suragch]

Description

This adds code documentation to the num for CSS units to help those not familiar with CSS.

I am one of those not so familiar with CSS. I wrote these comments with the help of AI and my own (possibly flawed) understanding, so please check them carefully for accuracy.

Type of Change

• 📝 Documentation

Ready Checklist

• [x] I've read the Contribution Guide.
• [x] In case this PR changes one of the core packages, I've modified the respective CHANGELOG.md file using the semantic_changelog format.
• [x] I updated/added relevant documentation (doc comments with ///).
• [ ] I added myself to the AUTHORS file (optional, if you want to).

[github-actions[bot]]

Package Version Report

The following packages have been updated: jaspr : 0.12.0 -> 0.13.0 jaspr_builder : 0.12.0 -> 0.13.0 jaspr_cli : 0.12.0 -> 0.13.0 jaspr_flutter_embed : 0.3.1 -> 0.3.2 jaspr_riverpod : 0.3.11 -> 0.3.12 jaspr_router : 0.4.1 -> 0.4.2 jaspr_test : 0.12.0 -> 0.13.0

[schultek]

LGTM, thanks for this contribution. 👍