Ignore charts repo URL in MD link checking

Somehow the GitHub Pages URL we use to publish and install the charts
works to install the charts in CI but also fails with a 404 in CI.

The URL is successfully used by Shipyard here:

https://github.com/submariner-io/shipyard/blob/
33dc2ffdadf6e520a2101368f7f7456c031bace0/scripts/shared/lib/
deploy_helm#L14

Signed-off-by: Daniel Farrell <dfarrell@redhat.com>

.markdownlinkcheck.json

@@ -5,6 +5,9 @@
     },
     {
       "pattern": "^http://localhost:"
+    },
+    {
+      "pattern": "^https://submariner-io.github.io/submariner-charts/charts"
     }
   ]
 }