Add automation to block PRs which modify `azuredeploy.json` for bicep-driven quickstarts

The author of #13525 wasn't aware this was a Bicep-driven sample, and only modified the `azuredeploy.json` (which was then overwritten by automation). We should catch and block this in automation to make the PR process more user-friendly.

Start by locating the repository's PR automation and the logic that identifies Bicep-driven quickstarts. Reproduce the scenario from #13525, where only azuredeploy.json is modified, and define the expected blocking behavior. Done when such PRs are blocked with a user-friendly explanation and the existing automation checks pass.

Written by the indexing model from the issue text.