Create subscription for 'Azure subscription as an Event Grid source'

[Create Event Grid subscription for resource events](https://github.com/Azure/azure-quickstart-templates/tree/master/101-event-grid-resource-events-to-webhook)

### Issue Details

The readme on the template above says `This template creates Event Grid subscription for either a resource group or Azure subscription` but it always creates a resource group subscription. We can see this on Azure portal on Event Subscriptions, that the subscription appears only on `Topic Type: Resource Groups`.

The ARM template [documentation](https://docs.microsoft.com/en-us/azure/templates/microsoft.eventgrid/2019-01-01/eventsubscriptions#property-values) only says

```
Use Microsoft.EventGrid/eventSubscriptions when subscribing to resource group events. Use //providers/eventSubscriptions when subscribing to custom events or events for supported resource publishers.
```

But it has nothing on how to subscribe to [Azure subscriptions events](https://docs.microsoft.com/en-us/azure/event-grid/event-schema-subscriptions) for resources across the subscription. How do we create such a subscription using ARM templates?

Start with the 101-event-grid-resource-events-to-webhook template and its README, then compare the declared Event Grid resource scope with the linked ARM template documentation. Verify how the template currently creates a resource-group subscription and determine the required subscription-level form. Done means the template supports the stated Azure subscription scope and the README accurately describes it.

Written by the indexing model from the issue text.