docs: Recommend use of Workload Identity for Azure Key Vault (#2906)

* docs: Recommend use of Workload Identity for Azure Key Vault

Mentions AAD Pod Identity is deprecated and updates overview
of supported authentication modes for Azure Key Vault.

This removes "should use aad-pod-identity" wording, see
https://github.com/external-secrets/external-secrets/discussions/2901

Signed-off-by: Mateusz Łoskot <mateusz@loskot.net>

* docs: Fix missing link to Multi-Tenancy Guide

Signed-off-by: Mateusz Łoskot <mateusz@loskot.net>

* docs: Fix typos

Capitalise own names.

Signed-off-by: Mateusz Łoskot <mateusz@loskot.net>

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Signed-off-by: Mateusz Łoskot <mateusz@loskot.net>