linkcheck.yml: add CI job that checks markdown links

Closes #8158

.github/workflows/linkcheck.yml

@@ -0,0 +1,23 @@
+# Docs: https://github.com/marketplace/actions/markdown-link-check
+
+name: Markdown links
+
+on:
+  # Trigger the workflow on push or pull requests, but only for the
+  # master branch
+  push:
+    branches:
+    - master
+    - '*/ci'
+  pull_request:
+    branches:
+    - master
+
+jobs:
+  check:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@master
+    - uses: gaurav-nelson/github-action-markdown-link-check@v1
+      with:
+        use-quiet-mode: 'yes'