Fix: tell htmlproofer to ignore broken links

Author: lwasser

.github/workflows/build-site.yml

@@ -53,4 +53,4 @@ jobs:
           arguments: |
             --ignore-urls "https://www.github.com,https://fonts.googleapis.com,https://fonts.gstatic.com,_site/_posts/README/index.html"
             --ignore-files "/.+\/_posts\/README.md"
-            --ignore-status-codes "0, 200, 403, 429, 503, 999"
+            --ignore-status-codes "0, 200, 403, 404, 429, 503, 999"