docs: create KeptnApp reference page (#1391)

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

docs/content/en/docs/yaml-crd-ref/app.md

@@ -3,3 +3,137 @@ title: KeptnApp
 description: Define all workloads and checks associated with an application
 weight: 10
 ---
+
+`KeptnApp` defines a list of workloads
+that together constitute a logical application.
+It contains information about all workloads and checks
+that are associated with a Keptn application
+and a list of tasks and evaluations to be executed
+pre- and post-deployment.
+
+## Synopsis
+
+```yaml
+apiVersion: lifecycle.keptn.sh/v1alpha3
+kind: KeptnApp
+metadata:
+  name: <app-name>
+  namespace: <app-namespace>
+spec:
+  version: "x.y"
+  revision: x
+  workloads:
+  - name: <workload1-name>
+    version: x.y.z
+  - name: <workload2-name>
+    version: x.y.z
+  preDeploymentTasks:
+  - <list of tasks>
+  postDeploymentTasks:
+  - <list of tasks>
+  preDeploymentEvaluations:
+  - <list of evaluations>
+  postDeploymentEvaluations:
+  - <list of evaluations>
+```
+
+## Fields
+
+* **apiVersion** -- API version being used.
+* **kind** -- Resource type.
+   Must be set to `KeptnApp`
+
+* **metadata**
+  * **name** -- Unique name of this application.
+    Names must comply with the
+    [Kubernetes Object Names and IDs](https://kubernetes.io/docs/concepts/overview/working-with-objects/names/#dns-subdomain-names)
+    specification.
+
+* **spec**
+  * **version** -- version of the Keptn application.
+    Changing this version number causes a new execution
+    of all application-level checks
+  * **revision** -- revision of a `version`.
+    The value is an integer that can be modified
+    to trigger another deployment of a `KeptnApp` of the same version.
+    For example, increment this number to restart a `KeptnApp` version
+    that failed to deploy, perhaps because a
+    `preDeploymentEvaluation` or `preDeploymentTask` failed.
+  * **workloads**
+    * **name** - name of this Kubernetes
+      [workload](https://kubernetes.io/docs/concepts/workloads/).
+      Use the same naming rules listed above for the application name.
+      Provide one entry for each workload
+      associated with this Keptn application.
+    * **version** -- version number for this workload.
+      Changing this number causes a new execution
+      of checks for the Keptn application and the new version of the workload.
+  * **preDeploymentTasks** -- list each task to be run
+    as part of the pre-deployment stage.
+    Task names must match the value of the `name` field
+    for the associated [KeptnTaskDefinition](taskdefinition.md) resource.
+  * **postDeploymentTasks** -- list each task to be run
+    as part of the post-deployment stage.
+    Task names must match the value of the `name` field
+    for the associated [KeptnTaskDefinition](taskdefinition.md) resource.
+  * **preDeploymentEvaluations** -- list each evaluation to be run
+    as part of the pre-deployment stage.
+    Evaluation names must match the value of the `name` field
+    for the associated [KeptnEvaluationDefinition](evaluationdefinition.md)
+    resource.
+  * **postDeploymentEvaluations** -- list each evaluation to be run
+    as part of the post-deployment stage.
+    Evaluation names must match the value of the `name` field
+    for the associated [KeptnEvaluationDefinition](evaluationdefinition.md)
+    resource.
+
+## Usage
+
+Kubernetes defines
+[workloads](https://kubernetes.io/docs/concepts/workloads/)
+but does not define applications.
+The Keptn Lifecycle Toolkit adds the concept of applications
+defined as a set of workloads that can be executed.
+A `KeptnApp` resource is added
+into the repository of the deployment engine
+(ArgoCD, Flux, etc)
+and is then deployed by that deployment engine.
+
+You can create a `KeptnApp` resource as a standard YAML manifest
+or you can use the
+[automatic application discovery](../implementing/integrate/#use-keptn-automatic-app-discovery)
+feature to automatically generate a `KeptnApp` resource
+based on Keptn or [recommended Kubernetes labels](https://kubernetes.io/docs/concepts/overview/working-with-objects/common-labels/).
+This allows you to use the KLT observability features for existing resources
+without manually populating any Keptn related resources.
+
+## Example
+
+```yaml
+apiVersion: lifecycle.keptn.sh/v1alpha3
+kind: KeptnApp
+metadata:
+  name: podtato-head
+  namespace: podtato-kubectl
+spec:
+  version: "1.3"
+  workloads:
+  - name: podtato-head-left-arm
+    version: 0.1.0
+  - name: podtato-head-left-leg
+    version: 1.2.3
+  postDeploymentTasks:
+  - post-deployment-hello
+  preDeploymentEvaluations:
+  - my-prometheus-definition
+```
+
+## Files
+
+## Differences between versions
+
+The `spec.Revision` field is introduced in v1alpha2.
+
+## See also
+
+[Use Keptn automatic app discovery](../implementing/integrate/#use-keptn-automatic-app-discovery)