Add Markdown link check workflow

.github/markdown/markdown-links.json

@@ -0,0 +1,7 @@
+{
+  "ignorePatterns": [
+    {
+      "pattern": "^https://opensource.org/licenses"
+    }
+  ]
+}

.github/workflows/markdown-links.yml

@@ -0,0 +1,33 @@
+name: "Markdown links"
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+    branches:
+      - main
+  release:
+    types:
+      - published
+  schedule:
+    - cron: '0 18 * * 1'
+jobs:
+  check:
+    name: Check
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+    - name: Check links in modified Markdown files
+      if: github.event_name == 'pull_request'
+      uses: gaurav-nelson/github-action-markdown-link-check@v1
+      with:
+        base-branch: main
+        check-modified-files-only: yes
+        use-verbose-mode: yes
+        config-file: .github/markdown/markdown-links.json
+    - name: Check links in all Markdown files
+      if: github.event_name != 'pull_request'
+      uses: gaurav-nelson/github-action-markdown-link-check@v1
+      with:
+        use-verbose-mode: yes
+        config-file: .github/markdown/markdown-links.json