docs: Add customWelcomePage example to Custom NSIS script

[hueyyeng]

The docs doesn't cover customWelcomePage for NSIS installer as it is not possible to configure the Welcome Page using configuration.

This is useful if one need to display Welcome Page for NSIS installer as the default behaviour is intentionally omitted. Refer to #1501 (comment)

[changeset-bot]

⚠️ No Changeset found

Latest commit: f73da7c

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[netlify]

✅ Deploy Preview for car-park-attendant-cleat-11576 ready!

Name	Link
🔨 Latest commit	f73da7c
🔍 Latest deploy log	https://app.netlify.com/sites/car-park-attendant-cleat-11576/deploys/62cfbe0352992d0008de36b7
😎 Deploy Preview	https://deploy-preview-7001--car-park-attendant-cleat-11576.netlify.app/configuration/nsis
📱 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 site settings.