docs(cli): clarify that --debug-build-paths requires route group folders in paths (#92618)

<!-- CURSOR_AGENT_PR_BODY_BEGIN -->
### 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 -->
<!-- CURSOR_AGENT_PR_BODY_END -->

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---------

Co-authored-by: Cursor Agent <[email protected]>
Co-authored-by: Joseph <[email protected]>

docs/01-app/03-api-reference/06-cli/next.mdx

@@ -308,6 +308,9 @@ next build --debug-build-paths="app/page.tsx"
 # Build more than one route
 next build --debug-build-paths="app/page.tsx,pages/index.tsx"
 
+# Include route group folders in the path
+next build --debug-build-paths="app/(marketing)/about/page.tsx"
+
 # Use glob patterns
 next build --debug-build-paths="app/**/page.tsx"
 next build --debug-build-paths="pages/*.tsx"