[Documentation] Example for creating an ACR service connection using configuration file

[geverghe]

In the effort to automate the deletion and creation of ACR service endpoint, I've had hard time figuring out the following behaviors of the creation and removal of a service endpoint:

An app registration and service principal (as well as a role assignment) is created automatically upon creation of the service endpoint for ACR Container Registry When the service endpoint is deleted, the app registration / service principal and role assignment associated with that service endpoint don't get cleaned up

Issue reference from azure cli extension repo - https://github.com/Azure/azure-cli-extensions/issues/1497

Would be best to include an example in both az devops sevice-endpoint command example and https://docs.microsoft.com/en-us/azure/devops/cli/service_endpoint?view=azure-devops

[geverghe]

@ngnmhieu for visibility