Adding reference to PnP PowerShell (SharePoint#5168)

Adding in a reference to the documentation on the same topic using PnP PowerShell. Actual file is being added through PR SharePoint#5167

Author: KoenZomers

docs/declarative-customization/site-design-powershell.md

@@ -53,4 +53,5 @@ The following cmdlets are available for managing site designs and site scripts f
 - [REST API](site-design-rest-api.md)
 - [Apply a scope to your site design](site-design-scoping.md)
 - [SharePoint site design and site script overview](site-design-overview.md)
+- [SharePoint site design - PnP PowerShell commands](site-design-pnppowershell.md)
 - [SharePoint site design - Office 365 CLI commands](site-design-o365cli.md)