Add a event to trigger a workflow if a Template Repository is instantiated

Describe the enhancement

Provide a new event like these https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows that is triggered if a Repository is created from a Template Repository.

This would allow a Template author to automate initialization of a Repository. The workflow should run in the newly create Repository, not in the Template Repository.

Example when this would be useful

• I have a Template named "standard-nuget-package". Within this repository, there are files and folders named like "standard-nuget-package.csproj". This Template includes a workflow triggered on instantiation (maybe template_instantiated), that renames files, folders and replaces file contents to replace "standard-nuget-package" with the name of the newly created Repository, and commits the changes.
• Another user creates a new Repository named "MyCoolPackage"
• The workflow triggers and performs rename / replace operations, and commits these changes
• The new Repository is ready to go

I can image more of such initialization workflows that would benefit from this event.

Start by reading the linked GitHub Actions workflow events documentation and the issue's template-instantiation examples. Define the event's trigger and payload expectations, including that the workflow runs in the newly created repository rather than the template. Done means the event behavior and implementation scope are established, with validation for repository creation from a template.