Add workflow_dispatch event

This makes it possible to run the script to validate all links manually, via a button on the GitHub Actions panel. This will be useful for testing and when you need to run validation right away.

.github/workflows/validate_links.yml

@@ -1,6 +1,7 @@
 name: "Validate links"
 
 on:
+  workflow_dispatch:
   schedule:
     - cron:  '0 0 * * *'