Fix: add design docs for ResourceTracker (#2909)

* Fix: enhance rt logic and add docs

Signed-off-by: Yin Da <yd219913@alibaba-inc.com>

* Fix: test conflict

Signed-off-by: Yin Da <yd219913@alibaba-inc.com>

docs/examples/app-with-policy/apply-once-policy/apply-once.md

@@ -0,0 +1,31 @@
+# How to use ApplyOnce policy
+
+By default, the KubeVela operator will prevent configuration drift for applied resources by reconciling them routinely. This is useful if you want to keep your application always have the desired configuration in avoid of some unintentional changes by external modifiers.
+
+However, sometimes, you might want to use KubeVela Application to do the dispatch job and recycle job but want to leave resources mutable after workflow is finished. In this case, you can use the following ApplyOnce policy. 
+
+```shell
+$ cat <<EOF | kubectl apply -f -
+apiVersion: core.oam.dev/v1beta1
+kind: Application
+metadata:
+  name: apply-once-app
+spec:
+  components:
+    - name: hello-world
+      type: webservice
+      properties:
+        image: crccheck/hello-world
+      traits:
+        - type: scaler
+          properties:
+            replicas: 1
+  policies:
+    - name: apply-once
+      type: apply-once
+      properties:
+        enable: true
+EOF
+```
+
+In this case, if you change the replicas of the `hello-world` deployment after Application enters `running` state, it would be brought back. On the contrary, if you set the `apply-once` policy to be disabled (by default), any changes to the replicas of `hello-world` application will be brought back in the next reconcile loop.

docs/examples/app-with-policy/gc-policy/keep-legacy-resources.md

@@ -1,4 +1,4 @@
-## How to use garbage-collect policy
+## How to keep legacy resources
 
 Suppose you want to keep the resources created by the old version of the app. You only need to specify garbage-collect in the policy field of the app and enable the option `keepLegacyResource`.
 

docs/examples/app-with-policy/gc-policy/persist-resources.md

@@ -0,0 +1,79 @@
+# How to persist resources
+
+By leveraging the garbage-collect policy, users can persist some resources, which skip the normal garbage-collect process when application is updated.
+
+Take the following app as an example, in the garbage-collect policy, a rule is added which marks all the resources created by the `expose` trait to use the `onAppDelete` strategy. This will keep those services until application is deleted.
+```shell
+$ cat <<EOF | kubectl apply -f -
+apiVersion: core.oam.dev/v1beta1
+kind: Application
+metadata:
+  name: garbage-collect-app
+spec:
+  components:
+    - name: hello-world
+      type: webservice
+      properties:
+        image: crccheck/hello-world
+      traits:
+        - type: expose
+          properties:
+            port: [8000]
+  policies:
+    - name: garbage-collect
+      type: garbage-collect
+      properties:
+        rules:
+          - selector:
+              traitTypes:
+                - expose
+            strategy: onAppDelete
+EOF
+```
+
+You can find deployment and service are created.
+```shell
+$ kubectl get deployment
+NAME          READY   UP-TO-DATE   AVAILABLE   AGE
+hello-world   1/1     1            1           74s
+$ kubectl get service   
+NAME          TYPE        CLUSTER-IP      EXTERNAL-IP   PORT(S)    AGE
+hello-world   ClusterIP   10.96.160.208   <none>        8000/TCP   78s
+```
+
+If you upgrade the application and use a different component, you will find the old versioned deployment is deleted by the service is kept.
+```shell
+$ cat <<EOF | kubectl apply -f -
+apiVersion: core.oam.dev/v1beta1
+kind: Application
+metadata:
+  name: garbage-collect-app
+spec:
+  components:
+    - name: hello-world-new
+      type: webservice
+      properties:
+        image: crccheck/hello-world
+      traits:
+        - type: expose
+          properties:
+            port: [8000]
+  policies:
+    - name: garbage-collect
+      type: garbage-collect
+      properties:
+        rules:
+          - selector:
+              traitTypes:
+                - expose
+            strategy: onAppDelete
+EOF
+
+$ kubectl get deployment
+NAME              READY   UP-TO-DATE   AVAILABLE   AGE
+hello-world-new   1/1     1            1           10s
+$ kubectl get service   
+NAME              TYPE        CLUSTER-IP      EXTERNAL-IP   PORT(S)    AGE
+hello-world       ClusterIP   10.96.160.208   <none>        8000/TCP   5m56s
+hello-world-new   ClusterIP   10.96.20.4      <none>        8000/TCP   13s
+```