test: use Tailwind for inlineCss 'shared' mode test + update docs
- Use Tailwind CSS in root layout to test the main use case for 'shared' mode
- Use `await connection()` instead of `export const dynamic` to avoid conflict
with cacheComponents test
- Document the new 'shared' option in the inlineCss docs
- Remove outdated limitation about CSS duplication (now fixed)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>