Fix test-readme-links flakiness (#475)

Fix flakiness by verifying that the notebook exists at the referenced
path in the repository, rather than making a request to nbviewer, which
fails for rate limiting.

Author: gabrielfeo

.github/link-check-config.json

@@ -0,0 +1,10 @@
+{
+  "retryCount": 3,
+  "fallbackRetryDelay": "120s",
+  "replacementPatterns": [
+    {
+      "pattern": "^https://nbviewer.org/github/",
+      "replacement": "https://github.com/"
+    }
+  ]
+}

.github/workflows/test-readme-links.yml

@@ -24,3 +24,5 @@ jobs:
         with:
           folder-path: .
           max-depth: 1
+          use-verbose-mode: yes
+          config-file: .github/link-check-config.json