Examples: Add v1 deployment-chain example

examples/v1/deployment-chain/first/app.pipecd.yaml

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+apiVersion: pipecd.dev/v1beta1
+kind: Application
+spec:
+  name: deployment-chain-first
+  labels:
+    env: example
+    team: product
+  plugins:
+    # No input block: deployment.yaml + service.yaml are auto-discovered.
+    kubernetes: {}
+  postSync:
+    # Once this app finishes deploying, PipeCD triggers the next app in the
+    # chain (matched by application name). That app can then trigger the one
+    # after it, forming a sequence.
+    chain:
+      applications:
+        - name: deployment-chain-second
+  description: |
+    This app is the first link in a deployment chain. When it deploys successfully, PipeCD triggers deployment-chain-second, which in turn triggers deployment-chain-third - one commit rolls out three apps in order.\
+    References: [adding a new app](https://pipecd.dev/docs-v1.0.x/user-guide/managing-application/adding-an-application/), [app configuration](https://pipecd.dev/docs-v1.0.x/user-guide/managing-application/configuration-reference/)

examples/v1/deployment-chain/first/deployment.yaml

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+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: dchain-first
+  labels:
+    app: dchain-first
+spec:
+  replicas: 1
+  selector:
+    matchLabels:
+      app: dchain-first
+      pipecd.dev/variant: primary
+  template:
+    metadata:
+      labels:
+        app: dchain-first
+        pipecd.dev/variant: primary
+      annotations:
+        sidecar.istio.io/inject: "false"
+    spec:
+      containers:
+      - name: helloworld
+        image: ghcr.io/pipe-cd/helloworld:v0.32.0
+        args:
+          - server
+        ports:
+        - containerPort: 9085

examples/v1/deployment-chain/first/service.yaml

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+apiVersion: v1
+kind: Service
+metadata:
+  name: dchain-first
+spec:
+  selector:
+    app: dchain-first
+  ports:
+    - protocol: TCP
+      port: 9085
+      targetPort: 9085

examples/v1/deployment-chain/second/app.pipecd.yaml

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+apiVersion: pipecd.dev/v1beta1
+kind: Application
+spec:
+  name: deployment-chain-second
+  labels:
+    env: example
+    team: product
+  trigger:
+    # Allow this app to be triggered as part of a chain. onChain defaults to
+    # disabled, so downstream apps must opt in.
+    onChain:
+      disabled: false
+  plugins:
+    kubernetes: {}
+  postSync:
+    chain:
+      applications:
+        - name: deployment-chain-third
+  description: |
+    Second link in the deployment chain: triggered by deployment-chain-first, and once it succeeds it triggers deployment-chain-third.\
+    References: [adding a new app](https://pipecd.dev/docs-v1.0.x/user-guide/managing-application/adding-an-application/), [app configuration](https://pipecd.dev/docs-v1.0.x/user-guide/managing-application/configuration-reference/)

examples/v1/deployment-chain/second/deployment.yaml

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+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: dchain-second
+  labels:
+    app: dchain-second
+spec:
+  replicas: 1
+  selector:
+    matchLabels:
+      app: dchain-second
+      pipecd.dev/variant: primary
+  template:
+    metadata:
+      labels:
+        app: dchain-second
+        pipecd.dev/variant: primary
+      annotations:
+        sidecar.istio.io/inject: "false"
+    spec:
+      containers:
+      - name: helloworld
+        image: ghcr.io/pipe-cd/helloworld:v0.32.0
+        args:
+          - server
+        ports:
+        - containerPort: 9085

examples/v1/deployment-chain/second/service.yaml

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+apiVersion: v1
+kind: Service
+metadata:
+  name: dchain-second
+spec:
+  selector:
+    app: dchain-second
+  ports:
+    - protocol: TCP
+      port: 9085
+      targetPort: 9085

examples/v1/deployment-chain/third/app.pipecd.yaml

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+apiVersion: pipecd.dev/v1beta1
+kind: Application
+spec:
+  name: deployment-chain-third
+  labels:
+    env: example
+    team: product
+  trigger:
+    onChain:
+      disabled: false
+  plugins:
+    kubernetes: {}
+  description: |
+    Final link in the deployment chain: triggered by deployment-chain-second. It has no postSync chain, so the sequence ends here.\
+    References: [adding a new app](https://pipecd.dev/docs-v1.0.x/user-guide/managing-application/adding-an-application/), [app configuration](https://pipecd.dev/docs-v1.0.x/user-guide/managing-application/configuration-reference/)

examples/v1/deployment-chain/third/deployment.yaml

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+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: dchain-third
+  labels:
+    app: dchain-third
+spec:
+  replicas: 1
+  selector:
+    matchLabels:
+      app: dchain-third
+      pipecd.dev/variant: primary
+  template:
+    metadata:
+      labels:
+        app: dchain-third
+        pipecd.dev/variant: primary
+      annotations:
+        sidecar.istio.io/inject: "false"
+    spec:
+      containers:
+      - name: helloworld
+        image: ghcr.io/pipe-cd/helloworld:v0.32.0
+        args:
+          - server
+        ports:
+        - containerPort: 9085

examples/v1/deployment-chain/third/service.yaml

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+apiVersion: v1
+kind: Service
+metadata:
+  name: dchain-third
+spec:
+  selector:
+    app: dchain-third
+  ports:
+    - protocol: TCP
+      port: 9085
+      targetPort: 9085