Update TsVRC Core docs to match the new usage of Udon Test Kit

[ToniSeas]

What does this change

This PR replaces the documentation to correctly reference the usage of udon-test-kit

Checklist

• Commits are signed off (git commit -s) per the DCO — this certifies you wrote the change or otherwise have the right to submit it under this project's license.
• If AI tooling helped write part of this PR, that's disclosed in this description (see CONTRIBUTING.md), and you've personally reviewed, understood, and checked it renders correctly.
• npm run build passes locally (onBrokenLinks: 'throw' catches broken links this way).
• The change follows STYLE_GUIDE.md.
• If this documents tsvrc-core behavior, it matches what the code actually does.