*: check connectivity of external links via GitHub action

.github/workflows/link-check.yaml

@@ -0,0 +1,34 @@
+name: 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: Link check for zh
+        uses: peter-evans/link-checker@v1
+        with:
+          args: -c 32 -d /github/workspace -r -v -x '(http://172.*|https://github.com/pingcap/dm/pull/.*)' zh*
+
+      - name: Link check for eh
+        id: lc
+        uses: peter-evans/link-checker@v1
+        with:
+          args: -c 32 -d /github/workspace -r -v -x '(http://172.*|https://github.com/pingcap/dm/pull/.*)' en*
+
+      - name: Fail if there were link errors
+        run: 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