Update navigating-pages.mdx

[evergreenx]

Why

This PR was created because the useRouter hook to navigate imperatively was not documented

How

Test Plan

Checklist

• [ ] Documentation is up to date to reflect these changes (eg: https://docs.expo.dev and README.md).
• [ x ] Conforms with the Documentation Writing Style Guide
• [ ] This diff will work correctly for npx expo prebuild & EAS Build (eg: updated a module plugin).