Change MD linkchecker to only run when md files are changed in a PR and to run nightly (#1148)

* Ignore null, timeout, status codes

* Only run when a markdown file is changed and run nightly

* Add markdown change

* Add run on link checker change and arbitrary change

.github/workflows/markdown-link-check.yml

@@ -4,6 +4,12 @@ on:
   workflow_dispatch:
   pull_request:
     branches: ["main"]
+    paths:
+      - '**.md'
+      - '.github/workflows/markdown-link-check.yml'
+      - '.github/.linkspector.yml'
+  schedule:
+    - cron: "0 0 * * *" # Run at midnight UTC daily
 
 permissions:
   contents: read