docs: Update redirects i18n documentation for App Router (#81347)
### What?
Updated the documentation to clarify that App Router requires manual
locale handling through dynamic route segments and middleware, with
examples showing how to implement i18n redirects correctly.
### Why?
The App Router doesn't use the i18n configuration in next.config.js like
the Pages Router does.
Closes NEXT-
Fixes #81227
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Co-authored-by: Joseph <joseph.chamochumbi@vercel.com>
Co-authored-by: Joseph <sephxd1234@gmail.com>