docs(cli): clarify that `--debug-build-paths` requires route group folders in paths (#92618)
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### What?
Adds a note to the `--debug-build-paths` section of the CLI docs
clarifying that [route
groups](https://nextjs.org/docs/app/building-your-application/routing/route-groups)
must be included in file paths.
### Why?
When using `--debug-build-paths`, users need to include route group
folders (e.g., `(marketing)`) in the file paths they pass. This isn't
obvious from the existing documentation — route groups are stripped from
URLs at runtime but must be present in file paths for this CLI option to
work correctly.
Addresses feedback from a docs comment noting that "Route groups are
also necessary, should be inferred from file paths, but it is not
straightforward from reading this."
### How?
Added a "Good to know" callout with an example showing how to reference
a page inside a route group:
```bash
next build --debug-build-paths="app/(marketing)/about/page.tsx"
```
<!-- NEXT_JS_LLM_PR -->
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Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: Joseph <icyJoseph@users.noreply.github.com>