Document decision tree about when to choose Az.Resources or Microsoft.Graph module for Azure AD user and group management

### Description

There are currently 4 PowerShell modules that can do user and group management in Azure AD:

* [MSOnline](https://docs.microsoft.com/en-us/powershell/module/msonline/?view=azureadps-1.0)
* [Azure AD PowerShell](https://docs.microsoft.com/en-us/powershell/azure/active-directory/overview?view=azureadps-2.0&preserve_view=true)
* [Microsoft Graph PowerShell](https://docs.microsoft.com/en-us/powershell/microsoftgraph/overview?view=graph-powershell-beta)
* AZ.Resources

I'm aware of the deprecation of the first two modules. Both point at Microsoft.Graph as a proper replacement, however I noticed that AZ.Resources also switched to Microsoft Graph lately. This all is pretty confusing. I think it should be properly documented when or not to choose for AZ.Resources instead of Microsoft.Graph, and when Microsoft.Graph should be preferred. If both modules serve a different audience, that should also be clear for people who need to make a decision for new projects and migration of old projects that use MSOnline or Azure AD PowerShell.

### Script or Debug output

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### Module versions

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### Error output

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