Add documentation for client router filter (#60585)
This adds documentation around the client router filter we leverage in
`pages` to allow incremental migration from `pages` to `app`. Also adds
mention of two experimental configs that can be useful for managing the
client router filter.
x-ref:
https://github.com/vercel/next.js/issues/47486#issuecomment-1889623898
Closes NEXT-2090
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Co-authored-by: Sam Ko <sam@vercel.com>
