Fix sporadic link check failure (#5515)

.github/workflows/ci.yml

@@ -56,7 +56,15 @@ jobs:
       - uses: actions/checkout@v3
 
       - name: Check markdown links
-        run: |
+        # using retry because of sporadic external site failures
+        uses: nick-invision/retry@v2.6.0
+        with:
+          # timing out has not been a problem
+          timeout_minutes: 15
+          # give external site some time to hopefully recover
+          retry_wait_seconds: 120
+          max_attempts: 5
+          command: |
             npm install -g markdown-link-check
             find . -type f \
                    -name '*.md' \