Add docs for MailRoom supporting alternative Azure deployments

What does this MR do and why?

Previously MailRoom could only be configured with Microsoft Graph for the default Azure deployment. However, there are currently three other national clouds supported in Azure (https://docs.microsoft.com/en-us/graph/deployments).

This commit documents support for the graph_endpoint and azure_ad_endpoint to enable support for these alternative Azure deployments.

Code MR: omnibus-gitlab!5978 (merged)

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