.github: add markdown link checker (#4513)

co-authored-by: Marko <marbar3778@yahoo.com>

.github/workflows/action.yml

@@ -0,0 +1,10 @@
+name: Check Markdown links
+on: push
+jobs:
+  markdown-link-check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@master
+      - uses: gaurav-nelson/github-action-markdown-link-check@0.6.0
+        with:
+          folder-path: "docs"