Add document to introduce the behavior of `konghq.com/protocol` on backend Services to configure protocol of translated KongService

[randmonkey]

Problem Statement

In #3750, we configure the protocol in translated KongService in hybrid gateways by konghq.com/protocol annotation of backend services. We should document the behavior and guide the user to properly use it.

Proposed Solution

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Additional Information

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Acceptance Criteria

• A guide page to tell users how to use the konghq.com/protocol to configure the protocol of translated KongService in the docs site