Backbone network add doc

To add of first version of documentation on the Helmholtz Backbone (ex VPN).
It's checked with Michael and Kars, so it's now fit for review.

To me it mainly needs:

• An overall check it contains the main items expected and is consistent with the rest of the doc (it's my first contribution) - @jandt-desy? (edit: in principle fine, see comments below)
• A few additional details on the 2nd use case - @wetzel-desy that'd be supi

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• Review app