Add first version of Network Topology documentation

docs/installation/environment-variables.md

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 # Environment Variables

docs/installation/network.md

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+
+# Network considerations and topology
+
+![Network topology](images/network_topology.excalidraw.svg)
+
+**External Loadbalancer:**
+
+The external loadbalancer is the most basic (and arguably important) piece of the network.
+It implements a consistent interface to incomming traffic from the outside world.
+
+It is generally recommend to assign it a static IP.
+This is IP is what is registered in DNS Records (A-Records).
+
+It is the first touch point for all third party initiated traffic.
+Hence it is accessed by
+
+- Users
+- Incomming Webhooks
+- Any sort of Rest API based communication
+
+**Cloud Router:**
+
+The internals of a Coud setup might be quite complex.
+
+As result, by default, outgoing traffic might not have transparent IP address.
+
+Most of the time this is not a problem at all.
+
+However, some third party services might rely on IP-Whitelisting.
+
+This applies to MPM-initiated communication.
+For example sending emails via a third party service.
+
+\*\*