Move expanded adapters docs to API reference (#92115)
## Summary
- move adapter implementation docs from
`app/api-reference/config/next-config-js/adapters` to
`app/api-reference/adapters`
- keep `adapterPath` as an entry page that points to the new adapters
section
- add matching Pages Router pointer docs under
`pages/api-reference/adapters`
- update existing docs links that referenced old `adapterPath#...`
anchors to the new adapters routes
