feat: App/Pages router for documentation

[kakenbutter]

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

It's frustrating to see that:

• there is a major lack of documentation for the App router, which has been out for quite a while now
• there isn't a toggle to easily switch between documentation
• pages with documentation for both the app and pages router are not consistent in how to switch between them (introduction page has toggle, folder structure has two separate pages)

The documentation has long been a problem with create-t3-app, but I hope that this can be resolved soon.

Describe the solution you'd like to see

App router documentation and a consistent, site-wide toggle for switching between the app and pages router

Describe alternate solutions

Leave the documentation as is

Additional information

No response