ci: allow manual check urls trigger (#76)

Author: holtgrewe

.github/workflows/check-urls.yml

@@ -7,6 +7,7 @@ on:
   pull_request:
     branches:
       - main
+  workflow_dispatch:
   schedule:
     #         ┌───────────── minute (0 - 59)
     #         │ ┌───────────── hour (0 - 23)