docs: add content to implementing/evaluations (#2073)

Signed-off-by: Meg McRoberts <meg.mcroberts@dynatrace.com>
Co-authored-by: Florian Bacher <florian.bacher@dynatrace.com>
Co-authored-by: odubajDT <93584209+odubajDT@users.noreply.github.com>

docs/content/en/docs/implementing/evaluations.md

@@ -3,3 +3,70 @@ title: Evaluations
 description: Understand Keptn evaluations and how to use them
 weight: 150
 ---
+
+A
+[KeptnEvaluationDefinition](../yaml-crd-ref/evaluationdefinition.md)
+resource contains a list of `objectives`,
+each of which checks whether a defined `KeptnMetric` resource
+meets a defined target value.
+The example
+[app-pre-deploy-eval.yaml](https://github.com/keptn/lifecycle-toolkit/blob/main/examples/sample-app/version-3/app-pre-deploy-eval.yaml)
+file specifies the `app-pre-deploy-eval-2` evaluation as follows:
+{{< embed path="/examples/sample-app/version-3/app-pre-deploy-eval.yaml" >}}
+
+The `evaluationTarget` is set to be `>1`,
+so this evaluation ensures that more than 1 CPU is available
+before the workload or application is deployed.
+
+This evaluation references the
+[KeptnMetric](../yaml-crd-ref/metric.md) resource
+that is named  `available-cpus`.
+This is defined in the example
+[metric.yaml](https://github.com/keptn/lifecycle-toolkit/blob/main/examples/sample-app/base/metric.yaml)
+file:
+{{< embed path="/examples/sample-app/base/metric.yaml" >}}
+
+To run an evaluation on one of your
+[Workloads](https://kubernetes.io/docs/concepts/workloads/)
+([Deployments](https://kubernetes.io/docs/concepts/workloads/controllers/deployment/),
+[StatefulSets](https://kubernetes.io/docs/concepts/workloads/controllers/statefulset/),
+[DaemonSets](https://kubernetes.io/docs/concepts/workloads/controllers/daemonset/),
+or
+[ReplicaSets](https://kubernetes.io/docs/concepts/workloads/controllers/replicaset/),
+you must:
+
+* Annotate your `Workloads`
+  to identify the `KeptnEvaluationDefinition` resource you want to run
+  pre- and post-deployment for the specific workloads.
+* Manually edit all
+  [KeptnApp](../yaml-crd-ref/app.md) resources
+  to specify the `KeptnEvaluationDefinition` to be run
+  pre- and post-deployment evaluations for the `KeptnApp` itself.
+
+See
+[Pre- and post-deployment checks](../implementing/integrate/#pre--and-post-deployment-checks)
+for details.
+
+Note the following:
+
+* One `KeptnEvaluationDefinition` resource can include
+  multiple `objective` fields that reference additional metrics.
+  In this example, you might want to also query
+  available memory, disk space, and other resources
+  that are required for the deployment.
+* The `KeptnMetric` resources that are referenced
+  in a `KeptnEvaluationDefinition` resource
+  * can be defined on different namespaces in the cluster
+  * can query different instances of different types of metric providers
+* All objectives within a `KeptnEvaluationDefinition` resource
+  are evaluated in order.
+  If the evaluation of any objective fails,
+  the `KeptnEvaluation` itself fails.
+* You can define multiple evaluations
+  for each stage (pre- and post-deployment).
+  These evaluations run in parallel so the failure of one evaluation
+  has no effect on whether other evaluations are completed.
+* The results of each evaluation
+  is written to a
+  [KeptnEvaluation](../crd-ref/lifecycle/v1alpha3/#keptnevaluation)
+  resource.