Add gitignore note for next-env.d.ts (#86809)

<!-- Thanks for opening a PR! Your contribution is much appreciated.
To make sure your PR is handled as smoothly as possible we request that
you follow the checklist sections below.
Choose the right checklist for the change(s) that you're making:

## For Contributors

### Improving Documentation

- [x] Run `pnpm prettier-fix` to fix formatting issues before opening
the PR.
- [x] Read the Docs Contribution Guide to ensure your contribution
follows the docs guidelines:
https://nextjs.org/docs/community/contribution-guide

### 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 #

-->

---
[Slack
Thread](https://vercel.slack.com/archives/C07BS1WEYAZ/p1764841040993409?thread_ts=1764841040.993409&cid=C07BS1WEYAZ)

<a
href="https://cursor.com/background-agent?bcId=bc-fc2eab83-628d-4ae1-b190-436d7e00063e"><picture><source
media="(prefers-color-scheme: dark)"
srcset="https://cursor.com/open-in-cursor-dark.svg"><source
media="(prefers-color-scheme: light)"
srcset="https://cursor.com/open-in-cursor-light.svg"><img alt="Open in
Cursor"
src="https://cursor.com/open-in-cursor.svg"></picture></a>&nbsp;<a
href="https://cursor.com/agents?id=bc-fc2eab83-628d-4ae1-b190-436d7e00063e"><picture><source
media="(prefers-color-scheme: dark)"
srcset="https://cursor.com/open-in-web-dark.svg"><source
media="(prefers-color-scheme: light)"
srcset="https://cursor.com/open-in-web-light.svg"><img alt="Open in Web"
src="https://cursor.com/open-in-web.svg"></picture></a>

---------

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

Author: icyJoseph

docs/01-app/01-getting-started/02-project-structure.mdx

@@ -31,22 +31,22 @@ Top-level folders are used to organize your application's code and static assets
 
 Top-level files are used to configure your application, manage dependencies, run proxy, integrate monitoring tools, and define environment variables.
 
-|                                                                              |                                         |
-| ---------------------------------------------------------------------------- | --------------------------------------- |
-| **Next.js**                                                                  |                                         |
-| [`next.config.js`](/docs/app/api-reference/config/next-config-js)            | Configuration file for Next.js          |
-| [`package.json`](/docs/app/getting-started/installation#manual-installation) | Project dependencies and scripts        |
-| [`instrumentation.ts`](/docs/app/guides/instrumentation)                     | OpenTelemetry and Instrumentation file  |
-| [`proxy.ts`](/docs/app/api-reference/file-conventions/proxy)                 | Next.js request proxy                   |
-| [`.env`](/docs/app/guides/environment-variables)                             | Environment variables                   |
-| [`.env.local`](/docs/app/guides/environment-variables)                       | Local environment variables             |
-| [`.env.production`](/docs/app/guides/environment-variables)                  | Production environment variables        |
-| [`.env.development`](/docs/app/guides/environment-variables)                 | Development environment variables       |
-| [`eslint.config.mjs`](/docs/app/api-reference/config/eslint)                 | Configuration file for ESLint           |
-| `.gitignore`                                                                 | Git files and folders to ignore         |
-| `next-env.d.ts`                                                              | TypeScript declaration file for Next.js |
-| `tsconfig.json`                                                              | Configuration file for TypeScript       |
-| `jsconfig.json`                                                              | Configuration file for JavaScript       |
+|                                                                              |                                                                                    |
+| ---------------------------------------------------------------------------- | ---------------------------------------------------------------------------------- |
+| **Next.js**                                                                  |                                                                                    |
+| [`next.config.js`](/docs/app/api-reference/config/next-config-js)            | Configuration file for Next.js                                                     |
+| [`package.json`](/docs/app/getting-started/installation#manual-installation) | Project dependencies and scripts                                                   |
+| [`instrumentation.ts`](/docs/app/guides/instrumentation)                     | OpenTelemetry and Instrumentation file                                             |
+| [`proxy.ts`](/docs/app/api-reference/file-conventions/proxy)                 | Next.js request proxy                                                              |
+| [`.env`](/docs/app/guides/environment-variables)                             | Environment variables (should not be tracked by version control)                   |
+| [`.env.local`](/docs/app/guides/environment-variables)                       | Local environment variables (should not be tracked by version control)             |
+| [`.env.production`](/docs/app/guides/environment-variables)                  | Production environment variables (should not be tracked by version control)        |
+| [`.env.development`](/docs/app/guides/environment-variables)                 | Development environment variables (should not be tracked by version control)       |
+| [`eslint.config.mjs`](/docs/app/api-reference/config/eslint)                 | Configuration file for ESLint                                                      |
+| `.gitignore`                                                                 | Git files and folders to ignore                                                    |
+| `next-env.d.ts`                                                              | TypeScript declaration file for Next.js (should not be tracked by version control) |
+| `tsconfig.json`                                                              | Configuration file for TypeScript                                                  |
+| `jsconfig.json`                                                              | Configuration file for JavaScript                                                  |
 
 <AppOnly>