feat: Add a pages/app router tab to nextAuth.js

[JayaDeHart]

Is your feature request related to a problem? Please describe.

The current nextAuth documentation page: https://create.t3.gg/en/usage/next-auth is structured around the pages router. With create-t3-app moving to the newest version of next, this documentation will be inaccurate for people using the most recent version.

Describe the solution you'd like to see

A tab at the top to switch between the existing version of the page for people still using the pages router, and updated documentation for using next-auth 5 (auth.js) with the app router / next-15. I've seen that the PR to introduce next-15 and next-auth 5 is currently in progress, so perhaps this could wait until that is finished.

Describe alternate solutions

Alternatively, we could just replace the current content with updated docs, since most people visiting the site for documentation will be using the most recent version.

Additional information

I am happy to make a PR that addresses this issue and adds the new documentation in.