HOTFIX: pages doc `next-config-js` was moved from `03` to `04` (#68370)
### Why?
`types-and-precompiled` is failing on CI:
https://github.com/vercel/next.js/actions/runs/10186089792/job/28178038314
### How?
The script `validate-externals-doc.js` was validating the
`docs/.../03-next-config-js/...mdx` page, but the page was moved to
`docs/.../04-next-config-js/...mdx` in
https://github.com/vercel/next.js/pull/67839