Clarify Azure Front Door's default caching behaviour in PoP

Hi,

I have been going through the following 101 quickstart example:
https://github.com/Azure/terraform/tree/master/quickstart/101-front-door-premium-storage-blobs-private-link

In my project we have a similar setup where we serve a single-page app (SPA) through Azure Front Door (Standard/Premium) from storage account.

But what caught my attention is the default caching suggested by the diagram and data flow description (steps 3 & 6).

The Terraform resource that manages caching is `azurerm_cdn_frontdoor_route` and the [documentation](https://registry.terraform.io/providers/hashicorp/azurerm/latest/docs/resources/cdn_frontdoor_route#cache) states that if the `cache` block is not defined in the resource the built-in cache in Azure Front Door should be disabled for the route.

My question: is there some level of default caching taking place in Azure Front Door even if the `cache` block is not defined for a route in Azure Front Door? And if this really is so could be made more clear in the 101 example as well as in Terraform azurerm documentation?

Review quickstart/101-front-door-premium-storage-blobs-private-link and the azurerm_cdn_frontdoor_route cache documentation. First establish the documented default behavior for a route without a cache block, then update the relevant example and documentation so the caching behavior is explicit.

Written by the indexing model from the issue text.