disable email checks on markdown-link-check (#4461)

markdown-link-check.fast.json

         {
             "pattern": "^https?://",
             "comment": "Skip external links for fast runs."
+        },
+        {
+            "pattern": "mailto:",
+            "comment": "mailto checking broken"
         }
     ]
 }

markdown-link-check.full.json

         {
             "pattern": "https://forum.unity.com/forums/ml-agents.453/",
             "comment": "Not reachable from github action"
+        },
+        {
+            "pattern": "mailto:",
+            "comment": "mailto checking broken"
         }
     ]
 }