Document how to use `MachinePool`

# Description A minimal implementation of `MachinePool` will be added when https://github.com/kubernetes-sigs/cluster-api-provider-gcp/pull/1506 merges. - The book should have a dedicated section for how to use this new feature, including basic cluster configuration, limitations, etc. - Additionally, the experimental state of the feature should be documented, with the feature gate that must be set on the controller for it to be enabled.