TabGroup: Simpler underline style

[danielledeleo]

Proposed changes

Closes: nginxinc/docs-platform/issues/422

Notable changes:

• --magrin-tab-group typo
• padding changes
• no more border-bottom underline
• simpler inline styles in tabs.html

Before:

After:

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md and CHANGELOG.md)

[github-actions]

✅ Deploy Preview will be available once build job completes!

Name	Link
😎 Deploy Preview	https://frontdoor-test-docs.nginx.com/previews/nginx-hugo-theme/333/

---