Potential global name clash with ACR instance in quickstart

In the [quickstart ](https://github.com/Azure-Samples/explore-iot-operations/blob/main/quickstart/readme.md) to install Azure IoT Operations:

If you let the scripts autogenerate the name of the Azure Container Registry instance, then you're likely to get a global name clash in Azure, as they try to try to create an ACR called **iotopsclusteracr** which already exists.

As a workaround, make sure to add your own ACR instance name in the config file using the key: `container_registry`.

Start with quickstart/readme.md and the installation scripts it links to, then trace how the autogenerated container_registry value is chosen. Update the quickstart or generation logic so a fresh installation does not attempt the conflicting iotopsclusteracr name, and verify the documented container_registry workaround remains accurate.

Written by the indexing model from the issue text.