Add generic APPUiO Managed OpenShift 4 architecture page

docs/modules/ROOT/pages/references/architecture/index.adoc

@@ -0,0 +1,21 @@
+= APPUiO Managed OpenShift 4 Architecture
+
+Every APPUiO Managed OpenShift 4 has 3 control plane nodes.
+All APPUiO Managed OpenShift 4 clusters have https://docs.openshift.com/container-platform/4.13/nodes/nodes/nodes-nodes-creating-infrastructure-nodes.html[4 infrastructure nodes].
+Additionally, by default, APPUiO Managed OpenShift 4 clusters have 3 worker nodes.
+The minimum requirement for each node are 4 vCPUs, 16 GiB of RAM and 100 GiB root disk.
+
+All APPUiO Managed OpenShift 4 clusters use https://cilium.io[Cilium] as the network plugin.
+APPUiO Managed OpenShift 4 uses the default OpenShift IP CIDRs for the pod and service networks: 10.128.0.0/14 for the pod network and 172.30.0.0/24 for the service network.
+There's no standard IP CIDR for the node network ("machine network" or "cluster network) for all APPUiO Managed OpenShift 4 clusters.
+However, a default IP CIDR for the node network is defined for some infrastructure providers.
+
+TODO: diagram
+
+https://syn.tools[Project Syn] is used to configure APPUiO Managed OpenShift 4.
+Apart from deploying custom configurations for OpenShift components, Project Syn is also used to deploy multiple third party components, such as https://cilium.io[Cilium] and https://cert-manager.io[cert-manager] on each APPUiO Managed OpenShift 4 cluster.
+
+
+You can consult the architecture documentation for the supported providers for more details on how the exact architecture for a specific infrastructure provider looks like
+
+* TODO xref to provider arch

docs/modules/ROOT/partials/nav.adoc

@@ -4,7 +4,7 @@
 
 // TODO Consider to add a "Getting Started" section
 
-* Architecture
+* xxref:oc4:ROOT:references/architecture/index.adoc[Architecture]
 ** xref:oc4:ROOT:explanations/dns_scheme.adoc[]
 ** xref:oc4:ROOT:explanations/node_groups.adoc[]
 ** xref:oc4:ROOT:explanations/etcd_encryption.adoc[]