Document Pageable types returned in the Azure SDK for .NET packages

[scottaddie]

A new doc, with a title to be determined, should be created under the Azure SDK for .NET ToC node. The doc should:

• Use one of the Azure SDK Key Vault packages
• Replace this content
• Cover IEnumerable<T> vs. IAsyncEnumerable<T> in the context of the Azure SDK's responses
• Demonstrate using the System.Linq.Async package for LINQ operations with AsyncPageable<T>
• Not emphasize Reactive Extensions, but rather how to use the System.Linq.Async package with the Azure SDK for .NET

Please work with @pakrym for any engineering questions related to this doc.

[IEvangelist]

Additional consideration(s):

• Potential parity with the existing Azure SDK for Java article on the same subject