Add gitignore note for next-env.d.ts (#86809)
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### What?
Adds a note to the `next-env.d.ts` entry in the project structure
documentation.
### Why?
To clarify that `next-env.d.ts` is a generated file and should be
excluded from version control (i.e., added to `.gitignore`). This helps
developers follow best practices for Next.js project setup.
### How?
Updated `docs/01-app/01-getting-started/02-project-structure.mdx` to add
"(should not be tracked by version control)" to the description of
`next-env.d.ts`.
Closes NEXT-
Fixes #
-->
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