Update Kong Vaults usage guide to describe KongVault CRD

[czeslavo]

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Problem Statement

Update the Kong Vaults usage guide to explain how to use the new KongVault CRD.

Proposed Solution

• Currently the guide includes only static configuration on the Gateway deployment level - supplement the guide with alternatives that configure the same vaults but using KongVault CRD
• Supplement the guide with examples of referring to vault-stored secrets in Plugins configuration

Additional information

No response

Acceptance Criteria

• As a user who wants to use secrets stored in Kong-supported vaults, I'm aware I can configure access to them using the new KongVault CRD