Update getting started docs to clarify use of --experimental-app flag

[grassfieldk]

The --experimental-app flag was required for enabling the App Router in early versions of Next.js 14, but is no longer necessary in recent version.

[netlify]

✅ Deploy Preview for contentlayerwebsite ready!

Name	Link
🔨 Latest commit	e263f1b
🔍 Latest deploy log	https://app.netlify.com/projects/contentlayerwebsite/deploys/6885845093eea90008e4f695
😎 Deploy Preview	https://deploy-preview-122--contentlayerwebsite.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[grassfieldk]

The --experimental-app flag was supported up to version 14, but in the latest version it has been removed and is now recognized as a directory name instead.

$ npx create-next-app@14 --typescript --tailwind --experimental-app --eslint contentlayer-example-14
✔ Would you like to use `src/` directory? … No / Yes
✔ Would you like to use App Router? (recommended) … No / Yes
✔ Would you like to customize the default import alias (@/*)? … No / Yes
Creating a new Next.js app in /home/grassfield/work/next/contentlayer-example-14.

$ npx create-next-app@15 --typescript --tailwind --experimental-app --eslint contentlayer-example-15
✔ Would you like your code inside a `src/` directory? … No / Yes
✔ Would you like to use App Router? (recommended) … No / Yes
✔ Would you like to use Turbopack for `next dev`? … No / Yes
✔ Would you like to customize the import alias (`@/*` by default)? … No / Yes
Creating a new Next.js app in /home/grassfield/work/next/--experimental-app.

For this reason, I have removed the flag.

[grassfieldk]

Come to think of it, the rest of the documentation assumes Next.js 14 or earlier, so just updating this section is not enough.
If you use Next.js 15, you would need to update all of the following steps as well, so it might have been better to use @14 instead of @latest.