Document or re-design NSG addition template because it has no real-world use cases

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[Add a subnet to an existing VNET](https://github.com/Azure/azure-quickstart-templates/tree/master/quickstarts/microsoft.network/subnet-add-vnet-existing)

### Issue Details

The template does not only add a new NSG into the existing subnet. It also removes existing settings e.g. delegation and service endpoints.

The template also requires passing the existing subnet address prefix as part of the deployment.

Those two features make the pattern used on this template impractical for real-world usage and it should be documented.

### Repro steps (*if necessary, delete otherwise*)
1. Deploy template with existing pre-configured subnet
2. Existing subnet properties are removed

### Possible fixes

* Document template and potential real-world usage issues?
* Analyze and document if it would be better to use Azure CLI or Powershell to attach NSG with the existing subnet
* Analyze and document if deployment script should be used to run CLI tool to attach NSG with the existing subnet

Start with the quickstarts/microsoft.network/subnet-add-vnet-existing template and reproduce its deployment against a pre-configured subnet. Check how the deployment removes delegation and service endpoints and why the existing address prefix is required. Compare documenting these limitations with using Azure CLI, PowerShell, or a deployment script; done means recording the real-world use cases and a justified recommended approach.

Written by the indexing model from the issue text.