[Doc] Add Gateway Integration Architecture Diagrams

[rootfs]

With all the gateways profiles integrated, let's enhance our documentation in the following way:

• Add integration architecture diagrams.
• Explain the added value to the gateways.
• Details about what functionalities are validated in e2e, and potentially link the CI status to the doc.

@Xunzhuo @srampal @szedan-rh @yehudit1987 @samzong

[samzong]

I'll take a first look at the current gateway integrations and the existing e2e coverage.

If it’s feasible for me, I’ll follow up and prepare a draft for review ~~~