[Docs] Add declarative peering section

[claudiolor]

Description

This PR add the documentation for the creation of peerings via a declarative approach, by applying CRs.

How Has This Been Tested?

N/A

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[frisso]

Suggested change

While offloading is independent from the network, which means that it is possible to negotiate resources and configure a namespace offloading without the inter-cluster network enabled, **a [working authentication configuration](#declarative-configuration-of-clusters-authentication) is a pre-requisite to enable offloading**.

While offloading is independent from the network, which means that it is possible to negotiate resources and configure a namespace offloading without the inter-cluster network enabled (either because it is not needed, or because is provided by a third-party mechanism), **a [working authentication configuration](#declarative-configuration-of-clusters-authentication) is instead a mandatory pre-requisite**.

[claudiolor]

I believe there might be cases in which inter-cluster network is actually not required (e.g. I need to offload some pods that do not require to "talk" with pods or services on the local cluster).
Maybe we can add a section somewhere (like in manual peering) where we explain the inter-dependency between the modules, and here add reference to that section.
Explaing everything here seems a little out of context to me, as I just wanted to say: "do not proceed with this guide if you didn't configure the authentication module"

[frisso]

Got it. Modified accordingly.

[frisso]

Looks very good to me.
Minor comments, mainly small rephrasing.