*: check connectivity of external links via GitHub action

.github/workflows/ci.yaml

@@ -17,9 +17,9 @@ jobs:
       with:
         config: './.markdownlint.yaml'
         args: '.'
-    - name: Verify links
+    - name: Verify internal links
       run: ./hack/verify-links.sh
-    - name: Verify link anchors
+    - name: Verify internal link anchors
       run: ./hack/verify-link-anchors.sh
     - name: "Check unclosed tags"
       run: 

.github/workflows/link-check.yaml

@@ -0,0 +1,37 @@
+name: external-link-check
+
+on: [pull_request]
+
+jobs:
+  base:
+    runs-on: ubuntu-18.04
+    steps:
+      - name: Check out code
+        uses: actions/checkout@v2
+
+      # NOTE:
+      #  - the check process can be very fast,
+      #    but if `-c` is too large then `Too Many Requests (HTTP error 429)` may be reported from `https://github.com/*`.
+      #  - we hardcode `--document-root` to `/github/workspace` in the container now.
+      #  - we use `http://172.*` as sample addresses in some docs, so we need to exclude them.
+      - name: Check external links in zh files
+        uses: peter-evans/link-checker@v1
+        with:
+          args: -c 32 -d /github/workspace -r -x "http://172.*|https://github.com/pingcap/dm/pull/.*|.*.md" zh*
+
+      - name: Check external links in en files
+        id: lc
+        uses: peter-evans/link-checker@v1
+        with:
+          args: -c 32 -d /github/workspace -r -x "http://172.*|https://github.com/pingcap/dm/pull/.*|.*.md" en*
+
+      - name: Fail if there were external link errors
+        if: ${{ steps.lc.outputs.exit_code }}
+        run: |
+          echo "Please check the broken links reported in previous steps \"Check external links in zh files\" and \"Check external links in en files\"."
+          exit ${{ steps.lc.outputs.exit_code }}
+
+      # Debug via SSH if previous steps failed
+      #- name: Set up tmate session
+      #  if: ${{ failure() }}
+      #  uses: mxschmitt/action-tmate@v2

en/maintain-dm-using-tiup.md

@@ -266,7 +266,7 @@ Execute `tiup list dm-master` to view the latest cluster version supported by Ti
 
 The process of using the `import` command is as follows:
 
-1. TiUP generates a topology file [`topology.yml`](https://github.com/pingcap/tiup/blob/master/examples/topology.dm.example.yaml) based on the DM cluster previously deployed using DM-Ansible.
+1. TiUP generates a topology file [`topology.yml`](https://github.com/pingcap/tiup/blob/master/examples/dm/topology.example.yaml) based on the DM cluster previously deployed using DM-Ansible.
 2. After confirming that the topology file has been generated, you can use it to deploy the DM cluster of v2.0 or later versions.
 
 After the deployment is completed, you can execute the `tiup dm start` command to start the cluster and begin the process of upgrading the DM kernel.