[ci] ignore researchgate not reachable in link check

.github/markdown_link_check_config.json

@@ -14,6 +14,9 @@
   "ignorePatterns": [
     {
       "pattern": "^https://opensource.org"
+    },
+    {
+      "pattern": "^https://www.researchgate.net"
     }
   ]
 }