Docs: Update `useLinkStatus` API reference (#78022)
Updating the docs after playing around with `useLinkStatus`:
https://github.com/vercel/next-app-router-playground/pull/184
Closes: https://linear.app/vercel/issue/DOC-4514/update-docs
Main changes:
- Update introduction
- Simplify example
- Add example for gracefully handling fast navigations (avoiding UI
flash)
