docs: add prefetchInlining config reference (#94650)
## What
- Adds a `prefetchInlining` `next.config.js` API reference page.
- Documents the default behavior and the `false` / object forms.
- Covers the `maxSize` and `maxBundleSize` thresholds and the
request-count vs deduplication trade-off.
- Links related navigation and prefetching docs.
## Why
`experimental.prefetchInlining` is currently only discoverable through
code, PRs, and release notes. This gives the option a canonical docs
page and addresses the missing docs report.
Closes #94458
## Duplicate check
I checked open PRs for `prefetchInlining` and `prefetchInlining docs`;
the current open matches are runtime/test work, not a config reference
page.
## Checks
- `pnpm exec prettier --check
docs/01-app/03-api-reference/05-config/01-next-config-js/prefetchInlining.mdx`
- pre-commit `lint-staged` (Prettier + ESLint)
Docs-only change; no runtime tests were run.
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Co-authored-by: Joseph <sephxd1234@gmail.com>
