docs: clarify layout.js nesting behavior in route groups (#95720)
### What?
- Adds one clarifying sentence (plus a link) to the "Organize routes
without affecting the URL path" section in
docs/01-app/01-getting-started/02-project-structure.mdx. No behavior
change, docs text only.
---
### Why?
- The existing sentence says you can add a layout.js inside a route
group folder to give it a different layout, but it doesn't say whether
that layout.js nests inside the top-level app/layout.js or replaces it.
Read in order, a reader hits this line before the page ever mentions
removing the top-level layout, so it's genuinely ambiguous at that point
in the page. The nesting behavior is only clarified later, in the
"Creating multiple root layouts" section — and even then, it's explained
for a different, more specific case (removing the top-level layout
entirely to give each route group its own root layout).
---
### How?
- Added one sentence stating that this layout.js nests inside the
top-level app/layout.js like any other layout by default, and linked to
"Creating multiple root layouts" for readers who want the root-layout
behavior instead.
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Co-authored-by: Joseph <sephxd1234@gmail.com>