Retire storage-keys-sas-tokens-code.md (deprecated KV-managed storage)

The underlying sharelink sample was retired from azure-sdk-for-net
(PR #57907) as KV-managed storage accounts are a legacy feature that
goes against Microsoft security guidance. Removed article, TOC/index
references, and added redirect to overview-storage-keys.

Co-authored-by: Copilot <[email protected]>

Author: msmbaldwin