docs: Add Machine Pool flag reference to GKE enabling docs

Prior to this change, a user would get an error when trying to provision
a quickstart GKE cluster with the provided instructions. The error would
instruct the user to enable the Machine Pool feature flag.

This change updates the readme for enabling capg with the GKE and
Machine Pool feature flags and enables users to provision quickstart GKE
clusters.

docs/book/src/topics/gke/enabling.md

@@ -1,9 +1,10 @@
 # Enabling GKE Support
 
-Enabling GKE support is done via the **GKE** feature flag by setting it to true. This can be done before running `clusterctl init` by using the **EXP_CAPG_GKE** environment variable:
+Enabling GKE support is done via the **GKE** and **Machine Pool** feature flags by setting them to true. This can be done before running `clusterctl init` by using the **EXP_CAPG_GKE** and **EXP_MACHINE_POOL** environment variables:
 
 ```shell
 export EXP_CAPG_GKE=true
+export EXP_MACHINE_POOL=true
 clusterctl init --infrastructure gcp
 ```