Add a workflow which checks if templated files are edited

[Cadair]

Describe the feature

I was thinking of ways we could gently encourage maintainers of the downstream templated packages to only make minimal required edits to the files managed by the template and change things here instead.

Proposed solution

Two things came to mind:

1. Use CODEOWNERS to request review of the template maintainers on PRs editing files managed by the template.
2. Make a GH Actions script which posts a comment on the PR saying "hey you edited a template file should these changes be made in the template"