Collection of app router migration guide tasks

I wanted to keep track on changes, so I will list the things I see missing in the current guide at https://blitzjs.com/docs/usage-next-13#app-directory

### Blocker

- [x] https://github.com/blitz-js/blitzjs.com/pull/848
- [x] https://github.com/blitz-js/blitzjs.com/pull/847
- [x] https://github.com/blitz-js/blitzjs.com/pull/845
- [ ] Explain that Blitz `Routes` don't work in app directory. Explain how to use the experimental types route feature of NextJS App Router and how to use this with `as Route` whenever a param has to be added.
See also https://discord.com/channels/802917734999523368/1161595039043047445
- [ ] TODO: Explain how to migrate `useParams`, `useRouter`, `useSearchParams`, see also https://discord.com/channels/802917734999523368/1161571344459239495
- [ ] TODO: How to work around that `next/head` is not available in App directory and `Metatags` are only available in server components which did not work for me (yet). We are considering moving back to ReactHelmet…
- [x] https://github.com/blitz-js/blitzjs.com/pull/844
- [ ] Double check the docs on authentication. All calls like `FooPage.authenticate = true` need to be replaces with `useAuthenticatedBlitzContext`. But first there is https://github.com/blitz-js/blitz/issues/4246 to be resolved.

### Improvements

- [ ] https://github.com/blitz-js/blitzjs.com/pull/851
- [ ] https://github.com/blitz-js/blitzjs.com/pull/846
- [x] https://github.com/blitz-js/blitzjs.com/pull/845

### Minor

- [x] https://github.com/blitz-js/blitzjs.com/pull/840
- [x] https://github.com/blitz-js/blitzjs.com/pull/843 => https://github.com/blitz-js/blitzjs.com/pull/844

Start with the App Router migration guide at https://blitzjs.com/docs/usage-next-13#app-directory and review each unchecked task against the linked Discord discussions and Blitz issue #4246. Done means the guide explains the remaining routing, navigation, metadata, and authentication migration points, while items blocked by #4246 are clearly identified.

Written by the indexing model from the issue text.