Add documentation for alias IP ranges feature

Author: jwmay2012

docs/book/src/SUMMARY.md

@@ -16,7 +16,9 @@
     - [Enabling](./clusterclass/enabling.md)
     - [Disabling](./clusterclass/disabling.md)
 - [General Topics](./topics/index.md)
+    - [Alias IP Ranges](./topics/alias-ip-ranges.md)
     - [Conformance](./topics/conformance.md)
+    - [GPUs](./topics/gpus.md)
     - [Machine Locations](./topics/machine-locations.md)
     - [Preemptible VMs](./topics/preemptible-vms.md)
 - [Developer Guide](./developers/index.md)

docs/book/src/topics/alias-ip-ranges.md

@@ -0,0 +1,33 @@
+# Alias IP Ranges
+
+Configure secondary IP ranges for instances via the `aliasIPRanges` field in `GCPMachineTemplate`.
+
+This enables CNI plugins like Cilium to use [Native Routing](https://docs.cilium.io/en/stable/network/concepts/routing/#google-cloud) by allocating pod and service IPs from the alias ranges.
+
+```yaml
+---
+apiVersion: infrastructure.cluster.x-k8s.io/v1beta1
+kind: GCPMachineTemplate
+metadata:
+  name: mygcpmachinetemplate
+  namespace: mynamespace
+spec:
+  template:
+    spec:
+      image: projects/myproject/global/images/myimage
+      instanceType: n1-standard-2
+      aliasIPRanges:
+      - ipCidrRange: /24
+        subnetworkRangeName: pods
+      - ipCidrRange: 10.96.0.0/16
+        subnetworkRangeName: services
+```
+
+The `ipCidrRange` accepts:
+- CIDR notation: `10.0.0.0/24`
+- IP address only: `10.0.0.1`
+- Netmask only: `/24`
+
+The `subnetworkRangeName` is optional and references a secondary IP range configured on the subnet.
+
+https://cloud.google.com/vpc/docs/alias-ip