Document support for Azure Blob sources

[travier]

Feature Request

Add documentation for #1923.

See:

• https://issues.redhat.com/browse/COS-2859
• https://issues.redhat.com/browse/RFE-1773
• provisioning azure: add remote ign ex on private azure blob fedora-coreos-docs#699

Environment

What hardware/cloud provider/hypervisor is being used to run Ignition? Azure

[travier]

I think we can close this now that we have the doc entry from #2012.