Add link checker

Author: philipbl

.github/workflows/link_checker.yml

@@ -0,0 +1,29 @@
+name: Build and check links
+
+on: 
+  push:
+  pull_request:
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: '3.0' 
+          bundler-cache: true 
+
+      - name: Build
+        run: make build
+          
+  link-checker:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: '3.0'
+          bundler-cache: true 
+      - name: Check links
+        run: make check_links