Document how to use Azure workload identities for the cache

Stan Hurequested to merge
sh-document-azure-managed-identities into main

What does this MR do?

In #38330 (closed), there was some confusion why cache uploads worked fine with Azure workload identities, but why cache downloads failed. This commit adds documentation about needing to add the azure.workload.identity/use pod label to both the runner manager and the runner pods.

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Edited by Stan Hu

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