[Documentation] Sovereign cloud usage examples and AzureCloudInstance enum

[GruberMarkus]

Documentation related to component
Authentication to sovereign clouds

Please check those that apply

• typo
• documentation doesn't exist
• documentation needs clarification
• error(s) in example
• needs example

Description of the issue
In PR #5671, support for sovereign clouds (Bleu, Delos, GovSG) has been added to MSAL.Net.

It would be great to have a usage example on how to authenticate against these clouds.

I would also like to see how the Microsoft.Identity.Client.AzureCloudInstance enum is connected to this, as the enum does not yet contain any hint to the new sovereig clouds.