add markdown-link-check action to github workflow

.github/workflows/docs-check.yml

@@ -0,0 +1,17 @@
+name: Check markdown links
+on:
+  push:
+    paths:
+      - '**.md'
+  pull_request:
+    paths:
+      - '**.md'
+jobs:
+  markdown-link-check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: gaurav-nelson/github-action-markdown-link-check@v1
+        with:
+          use-quiet-mode: 'no'
+          use-verbose-mode: 'yes'