Check broken links in markdown files (#224)

* Check broken links in markdown files
* use base branch config
* only check modified files
* run link checker only on pull request

.github/workflows/docs-lint.yaml

@@ -0,0 +1,20 @@
+name: Check Markdown links
+
+on:
+  pull_request:
+    paths:
+      - '**.md'
+
+jobs:
+  markdown-link-check:
+    name: Broken Links
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@master
+    - uses: gaurav-nelson/github-action-markdown-link-check@v1
+      with:
+        use-quiet-mode: 'yes'
+        use-verbose-mode: 'yes'
+        check-modified-files-only: 'yes'
+        base-branch: 'main'
+        config-file: '.github/workflows/markdown.links.config.json'

.github/workflows/markdown.links.config.json

@@ -0,0 +1,12 @@
+{
+    "ignorePatterns": [
+        {
+            "pattern": "^http://localhost"
+        }
+],
+    "timeout": "30s",
+    "retryOn429": true,
+    "retryCount": 3,
+    "fallbackRetryDelay": "30s",
+    "aliveStatusCodes": [200, 206]
+}