docs: Describe how to use ADLS instead of HDFS

[siegfriedweber]

Description

Describe how to use ADLS instead of HDFS

The ADLS support could also be properly implemented into the operator. The CRD could be extended and the operator could create the ConfigMap for ADLS. On the one hand, this would be a lot of work, on the other hand, this cannot be tested without an actual Azure storage account. Azure storage clones like Azurite do currently not support ADLS Gen2, see
Azure/Azurite#553. Adding a feature which cannot be integrated into Stackable's test landscape, is not sensible. And as the hurdle is low for the customer to define the ADLS ConfigMap themselves, only a documentation is provided.

Closes #539

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1. Code contains useful comments
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[fhennig]

LGTM (but I see you also requested lars' review)

[lfrancke]

Thanks Sigi. I agree with Felix: Looks good to me. And I agree with you. Let's not do a deeper integration for now. Thank you for the thorough work.