docs: add new e2e guide to book

Signed-off-by: Carlos Salas <[email protected]>

docs/book/src/SUMMARY.md

@@ -24,5 +24,6 @@
     - [Try unreleased changes with Nightly Builds](./developers/nightlies.md)
     - [Creating a cluster](./developers/cluster-creation.md)
     - [CI Jobs](./developers/jobs.md)
+    - [Adding new E2E test](./developers/e2e.md)
     - [Releasing](./developers/releasing.md)
 - [Roadmap](./roadmap.md)

docs/book/src/developers/e2e.md

@@ -0,0 +1,47 @@
+# Adding new E2E test
+
+E2E tests verify a complete, real-world workflow ensuring that all parts of the system work together as expected. If you are introducing a new feature that interconnects with other parts of the software, you will likely be required to add a verification step for this functionality with a new E2E scenario (unless it is already covered by existing test suites).
+
+<aside class="note">
+
+<h1>Tip</h1>
+
+You can find all logic and configuration files in the E2E folder of the repository [here](https://github.com/kubernetes-sigs/cluster-api-provider-gcp/tree/main/test/e2e)
+
+</aside>
+
+## Create a cluster template
+
+The test suite will provision a cluster based on a pre-defined yaml template (stored in `./test/e2e/data`) which is then sourced in `./test/e2e/config/gcp-ci.yaml`. New cluster definitions for E2E tests have to be added and sourced before being available to use in the E2E workflow.
+
+## Add test case
+
+When the template is available, you can reference it as a flavor in Go. For example, adding a new test for self-managed cluster provisioning would look like the following:
+
+```golang
+Context("Creating a control-plane cluster with an internal load balancer", func() {
+    It("Should create a cluster with 1 control-plane and 1 worker node with an internal load balancer", func() {
+        By("Creating a cluster with internal load balancer")
+        clusterctl.ApplyClusterTemplateAndWait(ctx, clusterctl.ApplyClusterTemplateAndWaitInput{
+            ClusterProxy: bootstrapClusterProxy,
+            ConfigCluster: clusterctl.ConfigClusterInput{
+                LogFolder:                clusterctlLogFolder,
+                ClusterctlConfigPath:     clusterctlConfigPath,
+                KubeconfigPath:           bootstrapClusterProxy.GetKubeconfigPath(),
+                InfrastructureProvider:   clusterctl.DefaultInfrastructureProvider,
+                Flavor:                   "ci-with-internal-lb",
+                Namespace:                namespace.Name,
+                ClusterName:              clusterName,
+                KubernetesVersion:        e2eConfig.GetVariable(KubernetesVersion),
+                ControlPlaneMachineCount: ptr.To[int64](1),
+                WorkerMachineCount:       ptr.To[int64](1),
+            },
+            WaitForClusterIntervals:      e2eConfig.GetIntervals(specName, "wait-cluster"),
+            WaitForControlPlaneIntervals: e2eConfig.GetIntervals(specName, "wait-control-plane"),
+            WaitForMachineDeployments:    e2eConfig.GetIntervals(specName, "wait-worker-nodes"),
+        }, result)
+    })
+})
+```
+
+In this case, the flavor `ci-with-internal-lb` is a reference to the template `cluster-template-ci-with-internal-lb.yaml` which is available in `./test/e2e/data/infrastructure-gcp/cluster-template-ci-with-internal-lb.yaml`.